I actually have done this. Our role reversal was actually quite non-dramatic - a much longer time span and with enough career choices and changes that it didn't feel all that unnatural to either of us. But my thoughts...
Make sure expectations all around are clear. Your new boss's expectations on your management of her, her expectations of you, your expectations of her.
It's worth a 1 on 1 with your boss before she gets back to make sure all is well with your mutual expectations on her return.
It's worth a 1 on 1 with her where you two talk about work assignments, and how you and she will resolve problems and what her scope of work will be.
I didn't do this so formally with my former boss, but we did have many conversations privately where we'd clarify with each other what we wanted or why we were sharing. We trusted each other enough that he could say "if I were you, this is how I'd take that" and I could ask "pretend to be me for a second... what do you think this means?" - he was actually a very valuable resource this way.
A key thing to keep track of here are motivations. If she's really uncomfortable with this - is it because she'd really rather be the boss? Did she ask for a demotion or get demoted? What's the motivation of the people that decided to rearrange your positions?
A thing you should aim to avoid here is getting in the middle of a discrimination lawsuit. Here's three scenarios, any could be valid:
- She asked for the demotion (even if she has regrets) because she wanted more time to be a mother
- She was demoted because she was never good at the job and in her absence the powers that be realized how awesome you are.
- She was demoted because the powers that be decided that as a new mother she wouldn't be able to do the job (LAWSUIT TERRITORY!)
If it's the first, then figure out whether you want to be a place she can vent - it's OK to say no, because it would be easy for her venting to feel to you like blame. Asking her to find another outlet, or a mentor who can help here is appropriate.
If she was bad at her job, and got demoted, you and your boss may want to talk about the expectations of performance management here. Asking a new manager to manage a poorly performing employee who is the former boss is asking a LOT and it's fair to ask for help in figuring out how to proceed.
If you have reason to believe that you are in lawsuit territory, proceed directly to HR and ask for help. If possible get yourself a legal consult from within the company.
Different people are better or worse and more or less comfortable with engaging on personal issues. I had a great boss who NEVER, EVER, EVER wanted to talk about my family life beyond the highest level of unemotional detail. She was still a great boss and a great mentor.
If you can't get into the touchy feely stuff without feeling profoundly awkward and uncomfortable, that's OK. Keep it about the work - welcome her back a work meeting, give her the time she deserves to help her ramp up and get guidance, keep the lines of commucation open. Be fair and balanced with how you treat the whole team in terms of giving time off for baby-related surprises. Everyone has a life, everyone has surprises, new motherhood/former-bossness shouldn't mean that you treat her unfairly compared to the rest of the team.
If you do like hearing baby stories, and being in touch with the personal lives of those you work with - go for it. It's OK, as long as you are not intrusive or judgmental. My experience has been that those that like personal connections are also pretty good at negotiating them.
Be Aware of...
What I think of as lingering bossiness. There's a power distance between employees and bosses. She's been the boss and she's used to having the deference and control that is now yours. Don't let her personality dominate meetings, don't let her take more than her fair share of your (or the team's) time and attention, don't let her have the control that is rightfully yours. If you would expect behavior from any other member of the team (please check with me before deciding on X) - expect it of her.
Give feedback clearly and directly (and mostly privately) when those points aren't met. Don't infer or imply the reasons for the problem (I know you used to be the boss... that's obviously why you are doing this..) - stick to the problem and what you need in terms of behavior change.
Provide a resolution system for emotionality
First - don't infer. New mothers are all different. Certainly new mothers have a very good chance of coming to work with new baby stories. But the nature of the experience as it is shared with coworkers is as individual as the people - I've had new moms be funny, sappy, sad, happy, frustrated, and almost anything else. As long as it stays within the bounds of culturally acceptable, it's just a part of who they are.
However if you have behavior that is not work appropriate, be prepared with a way to avoid the pattern of inappropriate behavior. Bursting into tears in a meeting, for example, can be greeted with a compassionate "do you need a minute? why don't you take a moment, take a step outside, collect yourself and come back". If it happens once, no big deal, if it happens weekly, it's a problem that you two should talk about.
Have some sense of what new mom facilities your company provides. Many these days provide "Mother's rooms" - so that new moms can ... deal with the biological impact of motherhood in privacy and peace. Learn where these are and if someone needs to get her a key to the room -- unless this is already common knowledge. Most mothers can deal with private mother stuff without having this be a serious work-performance impact. A male boss typically will not interrupt a woman making use of these private spaces, it's more appropriate to leave a sticky note on her monitor saying "come see me when you get a chance" and figure that you two will catch up in 15-20 minutes.
The rest of the team
There's bound to be some tension. Keep tabs on everyone and try to minimize the awkwardness. Be clear when you are (or are not) delegating authority to her, and when you want to decisions to filter exclusively through. Solicit the thoughts and opinions of others, they may have good ideas, or see elements to problems that you don't see.