Don't do them unless with very good person managing the whole set of the trip, better sober. The risks are not worth potential gains. Its kind of self-feeding - once you got paranoid/uncomfortable, it gets much easier to get into that place on further trips, since its usually just one mental muscle 'move'.

In contrary if you would manage to find a reliable good setter, you could move from that anxiety. But I don't know your details so take this with a massive pinch of salt, very unhappy childhood can mean many things including those that probably don't work well with psychedelics, ever.

I think "happy as a child" here really refers to awe and excitement around experiencing things for the first time. It is certainly not referring to reliving one's literal feelings from their personal youth.

That is an interesting hypothesis. I hope you are much happier as an adult.