The important context here is that in Germany you already have a right to reduced hours. You just don't have a right to getting your old hours back. This is important because the hours might very well have been given to other employees, permanently hurting your income and/or your career. (unless you find a different employer) Hence many new parents not daring to ask for reduced hours. This new demand/proposal by the union would result in a similar "re-employment guarantee" as those new parents who stop working completely for a number of months get already.

I mean currently it doesn't matter. We're always asked to up our contract to 40h again. The only question is if you go part time and then a crisis hits. In that case employers would be reluctant to bring you back up.