Even if you had 3 adults and only 2 kids, most times they will let you go together. But even if they don't, it's not something to worry about. 2 adults and 2 kids get on the plane and get started settling the kids in....the seats are 3 across, so have 1 of the parents take 1 kid and go to one side of the aisle....other parent and other kid take the other side....and both of you plop something in that third seat. When your other family member comes, they take whichever of the saved seats they want.....and if the plane is completely booked eventually some poor soul will take a seat next to you and your kid....but believe me, most will wait until there is no other choice than to sit next to a little one (even a well behaved one, this isn't a blast against kids...it's adults without kids that want to avoid them, lol). My little trick was always to leave the CENTER seat as the empty one. NO ONE likes those, lol. Then only when and if all other seats are full and someone needs that center seat...I move over from my aisle seat to be next to my kid who is at the window.
I know there are lots of fearful posts about being separated, but believe me, it's not something anyone else in the plane wants either. NO one is going to be separated from a child....occasionally, one of the parents will have to sit alone with the other parent and child together, but that's usually when it's a connecting flight and there wasn't much seat availability to begin with. I've been on a number of flights where the flight attendant will tell someone to move so mom and kid can sit together. I have also been on several flights where the attendant gets on the PA and says the plane ain't moving until mom and kid have seats together, lol. Those are always funny to watch these business suits sheepishly move...something they should have done in the first place. Yes, they may be paying more to get onboard first, but give me a break, separating kids and parents isn't good for anyone....do you really want to arrive at your business meeting after 4 hours of hearing that kid wail for his mom two rows behind him? No one does.