Sounds like your DD is too young to be interested in any of the great fie dining but you might try going to Chinatown - the two main roads are just north of Leicester Square. Restaurants there are good value, although staff are not usually particularly friendly. We live in central London and my 2 DC have loved Chinese duck and pancakes since they were very small - if you order it in a restaurant they always whizz the meat off the duck at the table and show you how to roll the pancakes which is a lovely bit of theatre.
There are also some amazing cake shops to go in - try Cutter and Squidge on Old Compton St, not far from Piccadilly Circus, for amazing layered cakes with butter icing in different flavours.
I'd avoid the chains if you can - mostly they are over-priced and very mediocre. If you want a nicer meal in an interesting environment you could try Portrait, on the top floor of the National Portrait Gallery which has wonderful views south. It's open evenings as well as through the day and you could comfortably go and have one courses so your DD wouldn't get bored. Pop into the Tudor gallery to see the portraits of the kings and queens, which is the most interesting bit of the gallery for DC.
Also the restaurant on the top floor (seventh) of Tate Modern has amazing views along the river - again you could go and just have one course. These are not badly priced for London - but be prepared to spend quite a bit while you are here. I'd advise you carry round bottles of water and maybe some apples or other fruit, and buy other "treats" on the go to re-fuel. There are some lovely independent little restaurants with wood fired pizza ovens and so on in the Covent Garden area too.
Its not always easy to find toilets when you need them - remember you can always walk into the free to enter museums to use loos, and many of the bookshops have a loo for customers.
Hope you have a lovely time.