Does it annoy me? Yes.
Do I see it as cutting? Yup.
Would I say something? Only to whomever I'm with.
Does it matter who's cutting? Wholeheartedly! A parent with 1 child joining another parent with another child would bother me less than, say, a family of 4 joining another family of 4. Then it's poor planning vs a "kid" issue like a potty emergency. I made the effort to get myself up and ready, my husband got himself up and ready, and somehow between the 2 of us, we managed to cajole a 7yo to get up and ready as well, *plus* grabbing a to-go breakfast because we made it a priority to get there early.... And when my grands see it, they call it out loudly. (Then we have a discussion about not everyone's person values being the same as ours.
All that being said, I have a MUCH bigger issue when 1 or 2 people are standing in line at a ride and 6 people join them right before we get on the ride. Had that happen a few months ago-- a middle-age mom waiting in line and then *6* teenagers come walking up, fresh slushies in hand, pushing by us (not even so much as an "excuse me"). That one burned my biscuits a bit as they literally extended our wait by 6 people!