We've not taken one that age to Disney, but to other "parks". In addition to the other info. the ladies gave, I'll share some of my "tips".
(1) get and bring a snuggli or sling. you aren't able to bring strollers in any of the lines, and with a snuggli or sling, you don't have to worry about baby being hit or jostled like you would if you were holding him/her. An added bonus, if you nurse, is that you can usually do so in relative privacy with a quick blanket over your shoulder with the snuggli or sling (I prefer the sling, myself
(2) bring one of those battery operated fans. Great way to keep you and baby cool
(3) in a ziploc bag, bring a damp wash cloth (great for cooling down as well as "clean ups"
(4) have a blanket handy, as the rides are often super-air-conditioned that time of the year
Otherwise, with my experience of DL, there should be many, many things your DS and his family should be able to do. In fact, there are fewer things I can imagine them NOT being able to do (such as Soarin, Space Mt., etc.) with the little one, as most everything is child/family friendly and they can do the swap.
One "note of caution", and I hesitate even saying this, since you ARE a grandma of 10

but if this is DS and wife's first child the issue may not be what they're "able" to do, but the dynamic of being nervous first time parents w/little bitty baby. If she's nursing, she may not be comfortable nursing in public, may have to stop and go somewhere private, etc., may be more nervious about the baby, generally, may take longer periods of time to change, feed, etc. And, she'll only be three weeks from HAVING the baby.
Maybe recommending a wheelchair might be a good idea, even, as then she can sit down instead of walking?
Anyway, best of luck and enjoy your trip.