First of all...
Macaws are pair bond birds. They can and will overbond with one person if allowed to...
Second, Red Fronts are generally the opposite of scarlets. Scarlets are MUCH more beaky. They are one of the harder big macs to work with.
I had another greenwing for about 17 years, and those birds pretty much grew up together, so they were fine. But that is an inexact science....
I have a red front and a young greenwing now. For the most part they get along, BUT my red front also is a very jealous bird at times... and will occasionally act pissy toward a bird that (if she wasn't so sweet and docile) could easily injure her... SO SUPERVISION IS A MUST. (Like sneak up and grab her tail when I'm paying attention to the greenwing and she wants to be picked up sort of thing...)
For the most part all my birds get along well. But big bird/little bird issues almost always end badly for the smaller bird.
As for big macs and children, that takes a significant amount of socialization work, and if the kid moves too fast, or makes the bird nervous, that usually ends badly for the kid's fingers....
Having said that, my daughter has been handling everything from Hyacinths, to large toos, to amazons, to conures... since she was 7 years old. BUT she was trained by some of the best bird handlers down at the rescue, and she grew up with birds... so... that is the exception rather than the rule.