Visit to grandparents cut short due to teenage behaviour

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I'm fully prepared to be told that AIBU, just want to gauge how others would have taken this.
DS14 and DD13 went down to see the in-laws for 4 days over the bank holiday period. About 4hrs on the train, plus cycling on our side and car ride on their side, so a considerable distance. 5yr old cousin who lives much closer to the in-laws was also visiting. They love seeing their cousin and playing with her, but from past experience can be pretty full on like toddlers are, excited to see their only cousins etc. After a couple of days of relentless playing (7am-10pm) DS and DD phones me, DD saying she is missing me, she gets emotional when tired, and DS saying he is not allowed a break from the playing. MIL overhears this and snaps at DS, bans him from dinner, further telling off etc etc, and ultimately they were sent home the next day as they had had enough of his moody, petulant, stone faced demeanour and refusal to apologise for saying telling us he needed a break. They were meant to come back the day after, instead they were taken to the station and dropped off, they came home at 9pm last night. Is this reasonable? I am totally on board with telling off for bad behaviour and feel that it's up to the grandparents to do that if necessary, DS can be like this sometimes, but in this instance, couldn't they have suck it up for a few more hours and sent them home the next day as planned? This is the first time ever that they have gone down without us, and according to the in-laws won't be happening again. Feeling sad for the kids mainly.
It's not the chastising I don't approve of, it's the way they were sent away abruptly.

If you're still reading replies OP I think I'd also be looking at limiting how much you tell these people about your challenges with your DC. Nothing wrong with needing to talk about your struggles and offload to another person but I wouldn't choose them. It sounds like they were using this information against your DS in a really unhelpful way.

My DD was upset because she was tired, that's usual for her. Even though she has fun playing with her cousin, it becomes too much after a couple of days on non-stop playing, and in the past I've been there and intervened when she's had enough.
DD said DS did nothing wrong and I believe her. His subsequent petulance is because of the way they were treating him.

They were down there for pleasure yes, but also because both DH and I were at work. So, whe. They came back early they were on their own the next day, no big deal at their ages, but it was a reason for the timing of the visit.

I don't know whether cousin overheard him, but he was on the phone to me and his exact words were "they won't let me have a break!", not that he has had enough of his cousin, he wouldn't have said anything like that knowing him well.

god that is bonkers expectation of two teenagers - not many adults could handle full on interacting with a 5 year old without down time - I have just been on holiday with a 5 year old and was tired after a few hours of it.

Teens need time to themselves!

Ironically, MIL said that DS was having lots of fun , hooting with laughter all day, in her words, so he was definitely playing with cousin and enjoying it, and he has a high threshold for such play considering his age, so they were lucky he played for that long! But it wasn't enough for them.

MIL anD FIL are really not accurate reporters of what went on. Everything you have described reeks of a narcissist/enabler set up. She is self centered and sees only what she wants to see and he obediently cleans up her mess after she gets angry.