have you visited Montage or Canvas & Cream? Montage has a separate room for children and Canvas & Cream doesn't usually attract many children.
The good thing about coffee shops in Forest Hill is there are so many, each catering for a slightly different market. if one seems full of children you can join them or move on to the next cafe.

on the original thread, I hope Hills and Parks does reopen as it is exactly what is needed in Honor Oak.