I would have suggested The Ivy House in Nunhead, which is very child friendly and runs various children's things in its ballroom.
However they often have live music in the ballroom on Sunday afternoons, though I don't know if they are at the moment.
Might be worth trying? It's a great room and would easily hold the numbers you say.
020 7277 8233
ETA: To forestall accusations of a vested interest, it's London's first community owned pub and I have community shares in it
Edited 1 time(s). Last edit was 2019:10:03:15:15:29 by Sue.