Oh, I remember just a few thing about this show. Maybe the grandpa was ISTP and the Grandma ESxJ, are'nt they? And I'm agree to type Arnold as an ISFP and Olga as an ESTJ or ENTJ (rather N I think). Their teacher is probably an ENFx.
Helga: ESTP (i thought this was obvious). but then again she is a little bit too romantic(atleast when dshe is alone) and poetic to be an ESTP. so i guess she could be a ENTJ ( a unhealthy one)