There's some very odd replies on this thread
I do sometimes wonder on these AIBU threads lately if some posters just enjoy being contrary just for the sheer hell of it, disagree by all means, but surely common sense still prevails & I really can't see why some posters are nit picking a perfectly reasonable worry 
Its NOT "normal" to leave a 19month old with people they barely know - it happens, you may do it, but that does not make it normal, nor in the best interest of the DC 
Nor is it disabilist to worry that a looking after a toddler MIL doesn't actually know well will be too much for her, as her injuries are not something she is used to dealing with & nor is this particular toddler - that's just common sense
smuggler I do think you should call your MIL & speak with her directly, but from the POV of concern for her & your DP taking the pee out out of his mum not thinking it through properly, as DD is at a bit of a handful at the moment & it must be hard enough for her to cope just looking after herself, let her know it really isn't necessary as you can make other arrangements & if she wants, maybe when she's feeling better she could have DD for a few hours another time - suggest DP visits her with DD after his GP visit, so she can see DD & DP can pull his finger out & help his mum when she needs it rather than be a selfish arse and take the piss see if she needs any help
as others say already, I suspect shes dreading it too & will be relieved but thinks her DS needs her help, so hasn't said no.
BTW I'm another of the disabled parents who will be offended by your post - more like offended by such nonsense replies 