I don't have kids, and if I did, I don't think I would be able to live the poly life I'm living now.
That said - even now, with all my time besides working hours my own, I find scheduling a challenge. I always seem to be able to find the time to see lovers and new interests, it's old friends, established relationships, and most importantly my relationship with myself, that suffers from being too busy.
a couple of things:
I try to never say 'I'm sooo busy'. I find it's a kind of self fulfilling thing. If I don't have time for something or someone I just say: I can't then, but I could do it then.
I try to say no every now and then. Very difficult especially with new and interesting and shiny relationships happening, but necessary.
I have lots of friends with kids who 'give' each other a certain amount of nights, evenings, days per month to do as they please while the other stays home with the kids. Could be anything from hanging out with friends to sitting alone in the library. For poly people this could be going on dates.
Cleo - forties straight female
Ren - husband of 20 years
Bo - BF of 1+ year
Lou & Jane - Ren's girlfriends
Lin - Bo's other GF