We have had in our town two indoor play centres with cafe facilities. Both failed financially (2001, 2003), just a little too dear? Should have been very successful, large play rooms with cafe tables around the sides. Happy to warm up baby milk or food; selection of meals for tots or adults. One of them had lots of creative marketing ideas, glue for sticking, playdough, and good food for reasonable prices. Exactly what my unable-to-sit-still-for-long children required. But they had a surcharge for the child's visit (£1/hour or £3 for unlimited hrs per child), much more than people were willing to pay. You get most of the same facilities for more time at a Mum+Tots group and just 50p (for as many children as you might have), which includes the biscuits & a cup of tea. And you see the same mothers each week, which means that you start to feel confident leaving them to watch your kids when you nip out to the loo; not so easy to do at a cafe.
I think the surcharge tends to be necessary to pay for the space; to be honest, cafes run on very tight margins because food is so cheap and commercial rates high; they need most of the floor area covered with tables, with most tables full most of the time, to really turn a profit. One of our local indoor-cafe-play areas moved into a church hall with correspondingly low rent, on the argument it was a community service they were offering. They still struggled to pay a living wage out. We are in a pretty middle class town, but I met many mums who didn't go to these play centres because they (the mums) tend to be earning little if anything. Eating out had become a bit of a luxury -- not something to waste on the stress of chasing children, anyway (because they still need lots of supervision).
If you still want to give the cafe a try make sure you have parking sorted out; poor parking contributed to the demise of the church-based play-cafe. That business now taken over as a non/low-profit thing, run by volunteer mums. They charge 50p/child/hr and reputedly are pretty busy. Only open 10am-3pm on Weds-Friday.
What does seem to succeed (commercially) are pubs or McD's (as opposed to cafes) with kiddy play areas.
Just recalling our leisure centre cafe has a small children's play area -- quite a good place to go, actually, but subsidised by the council, of course....And the food is lousy!! Some supermarket cafes have similar, but again, subsidised by supermarket.
Sorry to rain on your parade.