I think it's natural to be worried, but I also think you are going to have to accept that the longer you are apart, the more it's likely that you will start to make some decisions about the children independently from each other.
In this case, it doesn't sound like you have any real reason to be worried other than natural motherly concern. I would be careful of making a big deal of it (though to be fair you haven't actually said whether you intend to raise this with your ex or not) - or at least be very careful how you bring this up. If you push this issue, they could easily take it the wrong way, and it may work against you in sustaining a communicative relationship with your ex in the future.
FWIW, I know that my DP and his ex used to communicate a lot more closely with each other directly after their split and would share a lot with each other especially about what they were doing with their DS. Over time that communication has naturally waned, and whilst they are still on very good terms, they have learned to trust each other and I know they "check" things with each other much, much less than before. Including stuff about partners and overnights - my DP didn't check with his ex before I started sleeping over, though she had been told about my existence. Everything turned out fine, and me and DP and her and her new DP all get along great and regularly get together for social events and stuff to do with the various children.
I know that no two situations are the same, and if you really think there could be an issue then you could say something. But really, really be careful whether you truly believe this is a line your ex has overstepped, or whether it's something you should maybe try to deal with on your own.