What is it called when you lose a tooth but still have most remaining ones?
Literally, gap-toothed -
having a large space between two teeth
It's not endearing, and you probably wouldn't use it to describe a child in positive terms. It's often used as part of a description of a decrepit person.
A child's usually just "losing his/her teeth" unless you want a vivid depiction of a "gap-toothed smile."