In all likelyhood, starting next year I will work at reduced hours (at 60%) for some time as a form of parental leave. This is nothing I have to negotiate, the law here being what it is. I did roughly the same thing when my first kid was smaller, and while my boss was generally supportive he's no big fan of his people working less than full-time. I want to solve several problems:
The perception that I'm far less useful than when I'm fulltime.
The fact that I'm less useful than when working fulltime:
The projects we work on are usually big chunks, a week worth of fulltime work or more. So me being in the office less does not only mean that I do less work, but that one project will take longer to complete. We also often need to work with people outside the house, when someone has a question for me and I'm not around for a day or two this is an additional block.
Also, the individual projects we work on are highly dissimilar, so it's hard for a coworker to take over and cover for me. Also, we have to work on stuff as it comes up with about a month or two of rough planning ahead.
At another place I worked, part time was no problem as an individual task were smaller.
I want to go to my boss and say: "Hey boss, I know some of the work can be difficult with only part of the time. I have a few suggestions to ease the situation a bit, maybe we can talk about it soon so we still have time to make adjustments." But I don't have real ideas right now, where do I find any?
Edit to add
Looking at the answers so far, I want to stress that I don't have to negotiate or even ask for part time. This is parental leave, the law in germany is that they have to give me the time (and that I'm protected from beeing fired from 7weeks before the parental leave starts until it's end).
I also want to clarify that my parttime period will probably be half a year, my position is not so that a new hire could start right away and they would have to hire the replacement as a temp or move him/her to another department so I don't think they'll do that.