If the kids were at school/nursery as normal, they wouldn't have the dog or their toys around, so I'd start by asking their parents to remove those items from the room they're meant to be doing their 'learning' in and to tell the children why they're doing it and why you'll be there. If they don't do this, do it yourself when you arrive, letting the children know that you're there for their schooling, not playtime. They may be young but they need to understand the rules and to know that there will be other times they'll see you that ARE for playtime.
It's your son/DIL's responsibility to make sure the dog does its business in an appropriate place (outside) and if it is going to be indoors when you're there, you should have free reign to put it into the kitchen or utility so that it doesn't get too excited and do these things all over the place.
If you truly do have MS, you really don't need all the aggravation, this health condition is difficult enough to live with without having your 2 younger generations demanding inappropriately of you and I say this from the experience of being only 52 and having lived with MS for 43 years myself!
So, you need to talk to your son and DIL, tell them what you expect and what you're prepared to do as far as the children and their education and the dog are concerned, if this doesn't suit with what they want, tell them you're happy to see them at weekends or early evenings when they are there to control their menagerie themselves!