Unfortunately 3rd Jan is when the deluxe annual passholders can re-enter the park after about 3 weeks of being unable to visit so there will be a few people. Mind you the week after that will be even busier as the other annual passes become unblocked. I would suggest you pay for maxpass (or at least use fastpass) and also look into the
DAS (Disablity Access Service - see
https://disneyland.disney.go.com/au/plan/guest-services/guests-with-disabilities/ ) as this will help reduce the time spent waiting in lines (you still need to wait but it won't be in a line and you can rest in a quiet spot or look at the shops).
The good thing is you are there for 3 days so you can take it slow and if it gets too much then a break back at the hotel might be a good idea. The kids can get tired by mid afternoon and sometimes an afternoon rest allows them to return to the park for a few solid hours in the evening.