Yep, do it assertively with tone and body language.

I wouldn't even try to give an explanation. Just approach with open arms and say, "back to mummy now."

You'd have to be pretty brave to argue with that.

In which case, I'd do the, "no, you're coming back to mummy now aren't you DS"

So you're not actually talking to her directly IYKWIM.

Really, if she's too obtuse to take a hint then youv'e just got to be really direct.

Don't worry about giving her an explanation, only provide one if she arks up later and turns on the "oh, it's so unfair, I want to hold my grandson."

Sorry, I just get really cranky with insensitive people who have the rest of us running around in circles trying not to hurt THEIR feelings. It's all arse about.