OK. I'm going to bite. We are older than you, with 2 older (school age) kids. But mortgage free, with a similar income. We are very fortunate.
I would start by adding up a years worth of spend - including all the little "one offs" that add up. Then you can look at what is left.
900 sounds very low averaged across everything.
Roughly, we are:
Council tax: 200
Gas&electricity: 200
Phones, water, TV licence, other small regular bills:100
Kids activities/music lessons: 200 (we are past child care - have you factored this in?)
Everything else goes on the credit card that is paid off monthly. It is usually around 2000 a month. This included all food shops (750/month), petrol (250/month), ad hoc - meals out, birthdays, weekends away, clothes, car insurance/service/mot, house insurance, bits for the kids, bits for the house/garden etc etc.
Then 700 into DHs savings, 700 into my savings (we use these for eg new car, big renovations - new bathroom right now, and a summer holiday, along with filling a stocks and shares isa). And 700 into a private pension for me - DH has a brilliant one through work, but I have a 5 year gap and now a PT job, so my pension is lacking.
I'd work out exactly what is left, on average, each month, then increase your monthly splash cash, and share the rest between shorter term (ie cash) and longer term (pension, isa's) savings. Probably a quarter for day to day, a quarter for cash, quarter for isa and quarter for pension.