Putting rear hand brakes onto a 16" Diamondback Jr Viper I'm trying to add rear hand brakes onto a 16" bike for my son.  I've seen other online versions of the same bike model (Diamonback Jr Viper) with hand brakes; which makes me think I should be able to get it done.  Would anyone be willing to give me some advice for a good, easily activated hand break for a child (average sized 5yo, but on the weaker side of the spectrum). Of course, I know that I've only been talking about the handle; but, I would ask for your recommendations for the whole break system.  Planned use is very average; 1st bike, riding around the neighborhood...
It looks like to 20" version of the bike comes with a rear linear pull brake, but the 16" has a coaster brake. This is common practice with kids bikes as children young enough to be on a 16" bike don't have enough hand strength to operate a brake lever effectively.

I looked into doing the same thing when my daughter had a 16" bike, but found that it is not workable. In general there is no place to mount a brake caliper on a 16" bike. There is often a seat stay bridge with a hole in it but it is intended to mount a fender. The hole is too far away from the rim to mount a caliper.

My advice would be to save your money for the next bike you get your son when he grows out of the 16".