I don't travel over to Northcote Rd very often and my views are going to be pretty different than others aired in this thread - so take my post with a grain of salt, I guess.
I would absolutely love a cafe with some toys. BUT I don't want to see another pseudo soft-play slash cafe slash "some activities held here for an extra price" place.
If I want soft play I take the kids to Eddie Catz or Its A Kids Thing. Great places, but I can't afford to go as often as I would to a cafe. In a cafe what I want is 15-30 minutes of sitting around, drinking tea/coffee, having a nibble, maybe getting some reading done or chatting to a friend if I've come accompanied.
I loved the Crumpet on Bellevue Rd and went there often, although as others have pointed out they went downhill near the end with battered, dirty toys. Right now, my cafe of choice with young children is Belle Amie on Garratt Lane - they have great coffee and loose leaf tea, good pastries (haven't yet tried the food so can't speak to that) and the back room has some simple toys to play with.
It sounds like what you're proposing is a kind of hybrid between full-on soft play and somewhere like Crumpet/Belle Amie. I think you need to consider customers other than mothers with children - do you plan to cater to them as well? Take-out coffee for example? Wi-fi?
If I were to open a kid-friendly cafe (and I don't plan to - that's not my cuppa tea
), I would keep the play area small - maybe even not have a "play area" at all, but two "stations" the kids can float between to separate them a bit and prevent huddling. I'd have a train table with a ton of wooden trains (Thomas is almost always a hit with many children), and I'd have something like a wooden kitchen area with lots of play food.
I'd look at buying top-grade stuff, maybe even stuff designed for hospital waiting rooms or other similar high-traffic places, since it'll be very cleanable. No "loud" toys that will bother customers without children. And maybe a separate revenue-maker if you have the space - gumball machine or something, or one of those ride-on toys that cost 50p a go.
Best of luck in your venture