Both are great!
Hi haylmom, sorry I didn't get back to you sooner than this! Ah, your choice is like picking an ice cream flavor - "It's all good!"

You can't go wrong with either choice, happily. Both resorts are under common ownership and have much in common with regard to ski school structure (called "Perfect Turn") - both young childrens' programs are excellent.
At Sunday River, a fellow diva teaches kids! Why not try to pm her? -* backcountrygirl *- At Sugarloaf, the children's program is called "Mountain Magic." (DH used to teach older kids there, "Mountain Adventure") I've carried many a 3-4 year old onto the Whiffletree Superquad!

Has she skied before?
Both now have Magic Carpets and excellent kids' terrain. What kind of skiing do YOU prefer? Sunday River is vast and wide, lots of choices, more intermediate terrain. Sugarloaf is taller, more vertical, more ungroomed. Accommodations at Sugarloaf might be slightly more expensive. Sunday River draws a large Boston contingent and is marginally more crowded but disperses the crowds well with 16+ lifts.
It is currently DUMPING

- you needn't even think about whether either has good snow

- I live roughly halfway between the 2 resorts, and we are getting buried here right now.
What would your travel route be? Sugarloaf might be a bit closer, but either is still a long drive for you, so choose according to what you think you'd enjoy most. Since "College Week" begins at both resorts on January 2, do call one or the other ASAP, as it does get busy. Please let me know if you have any other further questions, either by forum or PM!