The food at the Rainforest Cafe is, IMHO, terribly overpriced. Kids enjoy the place, but you need to book in advance. They don't book all tables, but, if you can't find a slot, I'd recommend against showing up and waiting: 6 kids and 6 adults is quite a crowd, and you risk waiting a long time with kids becoming impatient. If it had been just one parent and one child it would have been different.
It might be easier to choose the after-lunch activity first (South kensington museum, ice rink, etc) and then the restaurant accordingly.