Turning patio to grass

If the patio was layed properly you will have flagstones and possibly sand and hardcore to remove from the garden , unless you can use it somewhere else, once you get down to soil level you will need to decide if you are laying turf or seeding , either way the ground will require to be turned over and all stones removed and raked before anything is put on top. It isnt as big a job as it sounds ,use a sack barrow rather than a wheelbarrow if you need to remove large flagstones from the garden. (or break them up)

One point I would add to what Maccrimmon has said is make sure you get the earth level. After you get to the soil and get the stones, etc out of it, it is going to settle unevenly. You might want to give it some time to settle and then level it off again before turfing/seeding.

I laid turf on soil that had been under a layer of gravel for several years and didn't give it time to settle after digging it up and raking, etc. The end-result looks fine from a distance, but is somewhat lumpy underfoot and hard to mow short because the blades take chunks out of it?! nothing quite like the feel of grass between your toes, tho. Your baby will love it. Good luck with the baby, by the way.