There are some excellent cartoons that teach languages, like Word World, but I'm having a hard time finding ones about math. There's of course Count von Count and other math-related characters on Sesame Street, but I'm looking for animated cartoons with mostly mathematical themes.
Schoolhouse Rock is always the top of the heap for multiplication. It worked in 1974 and still works frighteningly well today. As for addition and other basic forms; I'd suggest scouring YouTube. I've been very fortunate to find, not merely old Sesame Street counting; but other educational vids. (Note the answer from Dr. Stiehler above supports educational videos on YouTube)
Always pre-screen before showing them to little ones.
I always enjoyed the animations at "Brain Pop". They have a math section.
We watched Donald Duck in Mathmagic Land in my calculus class just for fun one day. I'm not a parent and only vaguely remember being the age you're probably targeting, but we thought it was amusing. It's pretty old (1959), but goes over some basic math concepts. It's only half an hour long, so it isn't really a cartoon series per se, but might be fun to show them once or twice.