Wasteland into a child safe family garden

Sounds like you need to start from scratch clear out all the waste even dig down to make sure any debris are clear then bring in some good quality soil especially if you want to grow vegetables

Unfortunately you've gotta start again. Once all the debris is removed and an even surface is achieved then it's always beneficial too remove 6 too 9 inches of the topsoil. This ensures all foreign bodies are removed such as glass,nails and rubble etc and also give you a sound deep bed for laying new lawn by rolls or seed and in your case growing veg. This sounds like a big job but too be honest the expense will come from the skips you'll have too hire. All other materials are pretty inexpensive. Hope this helps you out. Give me a shout if you fancy a quote.