It has to come from the recipient, or in the case of a very small child it has to feel like it came from the recipient. So a parent sending a text on behalf of a child doesn't feel right to me - for eg, I used to cut out the front pics of Christmas cards, DS would stick them to a bit of paper, and then do a wee scribble.
Now he's 8 writing lots of thank you letters is still a bit beyond him so he writes a generic one, I scan it and he signs each one.
If he had an email account, I would be perfectly happy for him to email, I loved OP's friend's video idea too. Basically, the DC has to have been involved in some way imo.