Disney doesn't allow children to ride on
ECV's with adults, although you will see people breaking the rule when CM's aren't looking.
But even if you could do it, it isn't safe. I've seen so many ECV drivers have to stop short, drive into a curb, or turn so quickly they tip over because of other guests getting in their way. It would not be safe to have a child on your lap or standing on the ECV while moving, even for the most experienced ECV driver.