Husbands tend to be a bit precious about sheds don't they?! Tell him he's got until the weekend to get it sorted or you'll find a cheap and cheerful one yourself and that'll be his Christmas (or whatever flavour holiday you celebrate, if any), Birthday, Father's Day etc. presents sorted for the next year or two.
The house is a mess so you and your husband need to tidy it, your MIL will only do a shit job anyway and certainly she cannot tidy your eldest's room without causing further upset.
The hedges are gone and all you can do here is encourage re-growth.
If the pumpkin plant is a goner then start a new growing project - avocado pits are good to watch, aloe plants grow quickly, Venus Fly Traps are fun, just move on and try to limit the sadness over the croaked pumpkin.
Your relationship with your MIL is now going to be strained for at least a while anyway; best not to make it worse so by phoning the police to report her for criminal damage or similar.
Obviously you aren't going to ask her to water the plants again - just like I no longer ask my FIL to feed our remaining fish.
I would put everything right and then one of you call MIL to ask why she has done what she has done - you never know, she could have been mid-tidying up and something happened with the rampaging two year old and she had to bolt, leaving the mess.
At least by asking why she has left the house a shit tip and ransacked the kids bedrooms you are raising the issue without confrontation.
Re the hedges just explain, again, that you keep them long for a reason and, although you appreciate her wanting to keep them trimmed, please do not cut them again.
Try to take it in your stride and not get too het up over it - lopped hedges and a few muddy footprints are frustrating yes, but not worth long term damage to your relationship with your MIL.