First I think you need to explore the humps, especially as you have a 2year old. They may be grass covered mounds of rubbish and not very safe.
Then it's like any garden. Do you want to grow veg? grow flowers? have it all as a playground or just a section?It is a lvely long garden and you could do lots.
There looks as ifyou have a greenhouse base, maybe you could get a cheap one for thee. You could split is up. into 3rds, or as might well have happened put veg down the side where the concrtete plinth is. You may well find the soil there is good and already been cultivated.
I would leave the bus and use it as a screen for a compost/grass cutting area behind it. You could of course tidy it up.
Make alist of what you want to do for all the family and then plan out a design and take small areas at a timeas and when you can afford to do more.
Any questions, feel free to post I'm always around