Was Pettygrew the antithesis to Snape?

Great question ! Never thought about this but yes, I think your daughter is right on !

We may add that while Severus is dang hot sexy, Wormtail is not. But maybe your daughter is still too young to think about that aspect.

However, I would think about a similarity between the two : both were nicknamed by James and Sirius.

As we're on the subject of the Marauders, 2 other oppositions between the 2 :
* Peter was friend with James and Sirius while Severus was not.
* Severus was way much more loyal to the Potters - while James was not his friend at all - and Peter was supposed to be James' friend and he betrayed him (while he was the Potters' Secret Keeper).

And I've got more :)

1) Peter is very short and fat, Snape is tall and thin
2) Peter and all his gang joined the order, Snape and all his gang joined the deatheaters.
3) Peter changed sides from the order to spy for the deatheaters, Snape changed sides from the deatheaters to spy for the order
4) Peter was not a good student, Snape was very good at school
5) Voldemort never trusted Peter's loyalty, Dumbledore always trusted Snape's loyalty
6) Peter hero worshipped James and Sirius, Snape hated James and Sirius
7) Peter had a squeaky voice, Snape had a soft low voice
8) Peter was a gryffindor who showed the worst slytherin points: deceiving, disloyal, selfish, Snape was a slytherin who sowed the best gryffindor points: bravery, loyalty, self sacrifice
9) Peter ended his life as a servant and seen as a lowly wizard, Snape ended his life as a headmaster and seen as a powerful wizard.
10) Everyone thought Peter was a hero but he turned out to be a villain, everyone thought Snape was villain but he turned out to to be a hero.

Can only think of one more:

Peter was a child's pet therefore under the care of a child and Snape was a teacher therefore a carer of children