I'm going to guess southern European MIL. Oops just read your last post, I was thinking Italy/Greece but it is a similar culture.
I know it is difficult but try to relax a bit. Talk to MIL about the Disney thing, say you thik it will be better if she waits until dn is older then gran can take both of them. Assuming dn is still young that will give you another 3 years or so to discuss it.
Your MIL probably has a hard time understanding you and your ways, sending a child to school at 5 in a uniform for a start, sending him to be 'early', ie before 10pm.
It may be annoying but it is totally natural for MIL to want both her grandchildren together, she is trying to recreate a Spanish family where all children and grandchildren visit grandma together.
Have you considered asking BIL and SIL to visit with dn? I'm assuming MIL would want to come along but if you do it when your ds has started school and it is not a holiday then you will get some time with dn. Or just SIL and dn, maybe an extended shopping trip something like that.
Everyone whose has married in to a different culture faces problems, you think you have the same culture or near enough and then BAM you upset someone by doing something completely normal (to you).
But your ds and any siblings will be able to juggle both cultures and have the benefits of both.
Fast forward a few years to when you have a sulky grumpy teenager and you can stick them on a flight to stay with family for the entire school holidays.
Try to remember it is cultural not personal. She is not going out of her way to hurt you, she is just behaving according to her culture.
I've known a few people in, for want of a better word, 'cross cultural' families. EG the family where mum has Xmas on the 25th December and the family go to her parents, and then on the 6th January it is dad's Xmas and they all go to his family. The kids get 2 Xmases and there is never an argument of your parents or ours.
Or the family where the adult son had to call a family meeting to explain that he had asked his girlfriend to marry him, she had accepted but, contrary to his culture, the wedding would not be in a few weeks but probably about 2 years away.
You probably feel a bit outnumbered in Spain and that can be depressing but if you do, look at your son, is he happy?