How big would the mini have to be, to be ridden by a very small child -- 2 year old?
The same as any other horse. I would wait until they're bodies are matured at 3 year old before training them to ride, drive or jump. However if you want to put very small children on them as with any horse the brain is more "aged" around 5 years old. They ARE horses... Just smaller. Everything is very much the same with them, handling, care, age of maturity etc.
As for 50 pound kids on 250 pound minis... To each there own opinion but my opinion is they're not really meant for riding. A child thats small enough is often to young to ride on there own, and minis move quick, choppy and bouncy so though they're cute they're not ideal as riding horses for little people. General rule of thumb for HORSES who are meant to carry the weight of a rider is 20% of there body weight, your average 1,000 pound horse carrying a 200 pound load rider + tack, Correct?
IF the same were to say of Miniature Horses a 250 pound mini could carry 50 pounds but I wouldn't ask a 32" tall horse to carry more than 30 pounds.... Just my opinion :)