for kids (i don't have/want any) i always suggest acekard because you can replace the firmware with a better one that lets you hide all the dangerous stuff like deleting games or whatever. takes a little work but editing an .ini file is a joke compared to the **** we do here

for myself i have a cycloDS since they are a european company and i hate cheap chinese junk that breaks. it's about $40 though.
i also bought a supercardDStwo for around $30, it's chinese but the company has been around since the original GBA and I trust them to release updates and do some decent product design. i would stick to one of those two companies unless you are really cheap, broke, or are certain your kid is going to loose the thing in a month. in that case just get the acekard, but try to buy it from somewhere reputable. last thing you want is some shady clone of a flashcart that bricks when you update it.
supercard dstwo can play divx apparently
cyclods is releasing a long awaited DSi version of their card in the next week, probably going to be the new gold standard.