It's the cutest thing, and they look perfectly comfortable, and will even go to sleep like this (only when being held) but, I was just wondering, is this normal? Is it okay?
Chickens cannot breathe easily when on their backs and can aspirate stomach contents (vomit). I do put them on their backs to dust them with poultry dust (someone told me that they had a chicken vomit when they did that).
Holding baby chicks that way is probably ok for a cute cuddle here and there but I'd not try to hold them like that routinely.
Not long ago, your chicks were inside their egg shells and were turned every which way for a month, so I doubt being upside down now is going to do them any harm.
You're on the right track handling them a lot. Chicks like yours who are cuddled and loved grow to be tame chickens that are delightfully easy to handle.