BEFORE plans are made for the Birthday party....
The Mom, should be included in planning her own son's Birthday party.
Did you ask her, if it was okay that you, plan it and when, instead of HER planning it?
The Mom, should be integral in her son's party or celebration.
It is up to her. And the Dad.
Certainly, because of the timing of his Birthday, can't the parents talk about it and per when the party can be or when the opening of presents can be, so that BOTH his parents have the chance to be there?
And maybe, the Mom, being she has him on Saturdays... MAY have plans ALREADY too, with her Son and HER family... too. That side of the family. Did you ask her???
So... it is needed, that SHE is consulted with, about HER plans with her son and her day with him, especially since this is also during the Holidays.
The Mom's side of the family and whatever plans she has with her family/relatives AND her son, is also important. This would be Christmas time. It does not matter, that the Mom's side of the family "never bring gifts" and thus they are not invited. The point is, that the boy and his Mom and their plans, are ALSO considered.
To me, this is her son, and the Mom, should be the key person planning any party for him, or his Dad or they together.
If my Mom, planned a party for my kids and did not consult with me at all about the timing or when or about the guest list or did not include certain relatives, I would be irked.
It is my, child. I have the honor of planning it for my own kids.
I am the Mom.
** The other concern is: It seems that your Grandson ALREADY knows, there will be a party for him. He is already excited. SO... if the Mom is consulted with about it and the plans of it, and she says "no...." to the details of the plans, then SHE will look like the "Bad Guy" in all of this. And it is not even her fault. And that is a shame. She is stuck now in a corner, because her son is already told there will be a party for him. But the Mom was not asked PRIOR, about when/if/how/what kind of party or about who is invited etc. Meanwhile, her Son was already told, that there is going to be a party for him.
The other thing is: Daddy usually has him Sunday afternoon thru Friday afternoon. This year, Christmas Eve is on a Monday and Christmas Day is on a Tuesday. HENCE, your Grandson will NOT be with his Mommy... for Christmas at all??? Hence, I would make sure, about any Birthday party plans for the boy, is per his Mommy's plans, too. Otherwise, it may be a Holiday where the Mommy and her side of the family, will NOT be able to even be with him. For Christmas time or his Birthday.
Even Thanksgiving this year, is on a weekday, a day when his Dad has him. Not his Mom. So his Mom is missing out on having her son... for the Holidays. Both Thanksgiving and Christmas. Unless the parents worked something out otherwise.
This all boils down to, the Mommy, having the key point in plans for her Son's party and the Holiday plans. It is her son and the holidays. I am sure any Mom would want to do things for and with her son, during this especially special time of year. It would have been nice if the Mommy got to tell her own Son that there will be a Birthday party for him, too.
If the party is on a Saturday, and that is Mommy's day to have her son, then yes, she can take him afterwards to her home. She may ALREADY have plans for him, and/or with her own family and friends on that Saturday. It is her day with her son after all. And there is nothing wrong with that. She has her time with her son.
But again, if there is a conflict of scheduling with the Mom per the party and just over the opening of gifts, and Mommy cannot allow her son to stay there on Saturday because it is her day with him & she already may have made plans for him and her that day, then SHE will look like the "Bad Guy" in all of this. And it is not her fault.