Does it look like there is any irritation / redness on her head? Or has your MIL been a bit rough with the lice treatment?

To get her body washed I'm thinkng maybe a shower cap might help - it'll keep the water off of her head anyway .

Maybe try to find some novel way to get DD to put the water on her head herself...like with one of those hose attachments or a special cup she can dip into the water and pour on her head....'with special water that has fairy dust in it which will give her special powers or something'??? (My DD is only 18mths so I'm not up to your stage yet, sorry - just trying to think outside the square! )...