It is probably both. Also they are approaching the new children with knowledge they didn't have the first time. At first it was a novelty and all was well. Then they realized little changes like how the schedule for the day was maybe more structured and less flexible or expectations for them like routine snack time not a snack whenever they asked. Now that new kids are starting part of it is oh no here we go again sort of thing.

Also your attention to them will be more remote such as holding a baby while helping them retrieve a ball that got wedged under the chair.

Just as we expect daycare children to come into care and adjust to the group compared to mom's undivided attention we also have to do the same for our kids. They don't have to like it and some of them really will not but it is their new reality too and we need to give them time to adjust. Try not to overcompensate to them for the disruption and instead treat it as just the new normal for everyone.

The ages of those coming in will matter too in the sense of what your own have to change - what toys are played with where, sharing of toys, waiting for you to be done with the other age group adn ready to do something with them, that sort of thing. Time is about all you can do and it will come to be normal for them or you will be able to see what the culprit is. Sometimes it takes one child being away or changing the schedule or a particular routine for it to all fall into place.