We are also taking an Alaskan Inside Passage cruise on HAL. We'll be on RYNDAM in August, also an extended-family cruise.
As long as your expectations are in line -- and they seem to be to me -- you should be fine. You will also have plenty of family onboard, which is another big plus. Our son's first cruise was at age 11 months. His second was at age 20 months, and we had his grandparents (my in-laws) along for that one, which was nice as they didn't live close by, and it also gave my husband and I some time to enjoy by ourselves. I was just looking at photos of that cruise yesterday, and it was so sweet to see him interacting with his grandparents, and how happy they all were on this trip.
Enjoy your planning! That's so generous of your Mom to treat the whole family to a cruise!