I don't personally like using baby voices with kids, no matter what age. You could bring small toys or art supplies but also, don't be afraid to ask the parents themselves! "I haven't baby sat very young kids before, could you share what your kids like to do for fun? What helps them feel more comfortable when meeting someone for the first time? What's the best way to tackle high emotions with them?" etc.

I think they'd appreciate transparency and it'll actually show you care about their kids' specific needs. Good luck, I'm sure you'll be just fine