Tokyo Parents+Kids Meet up for Fun

I am a new mother. Currently I live in Tokyo, Setagaya.
I thought it would be nice to get to know more mothers/parents with their kids/toddlers and to hang out with each other and our kids from time to time. Let our kids play with each other.
Fathers with their kids are also welcome.

I plan to do the meet ups in Baji Kouen (Private park) or Kinuta park only.
Not for too long at the moment. Up to 60-80min hang out, since I have a 1 year old and it takes time to come back home (20-30min).