I know you said the children are getting your mum's presents boxing day but is there any way to change that, could she give hers on christmas eve instead, then the PIL get the dissapointed faces and the hassle of changing. I suspect MIL wouldn't be doing this if she was in your mum's position of giving her present last.
Tell PIL if they insist on not telling you then they will need to keep the receipt as THEY might need to change present.
I don't think you are being a control freak at all and I don't think this is your mums fault either, of course she wants to get something that the child will like and hasn't already got. This is all just so unecessary on MIL part.
As for the person who said the children should learn to just say thank you, well yes I have taught my children this but really is it too much to ask that close family could confer, no matter how polite the child it will be obvious that the child is not really that excited by something they already have.
And sand's post:
Ours used to be "delivered by Father Christmas" so were all in the house on Christmas morning. My mum put her foot down early on to the inlaws (and her parents). It should be about the giving, not about feeling self satisfied with someone's reaction to the present you chose.
How very rude, your mum should be ashamed and any DIL of mine that tried this would be told in no uncertain terms that this would NOT be happening, of course it's all about seeing the childrens excited faces as they open the presents, your mum got to see that, that's why she wanted all the presents at her house on christmas morning, why on earth should she be the only one to see all the excited faces when she didn't even pay for those presents, shockingly rude!