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".
I liked @argenti-apparatus answer, but I just wanted to add that many children's bikes don't come with child size brake levers.
You may need to use the brake lever adjsutment screw to move the lever as close to the handle bar as possible as otherwise the child will not be able to physically use the brake.