I think you are probably fine for 3 of them. After age 16 you are travelling on an adult passport (or can be if the child passport is still valid), and that should allow free crossing. The other 2 you might want a letter, but the chance of being asked for kids that old is pretty low. My son, now 19, has been crossing the border on team buses for several years now with no letter.
I used to get travel letters when my kids were younger, but once they were old enough to answer for themselves, I stopped bothering and have never been asked for a letter. I was asked a situational question once when travelling with my then 13/14 yo son, but we just indicated that step-mom was flying and joining us a few days later once school let out for March break. Agents are taught to spot the BS, and if your story is straight and true it's easy sailing.
So really, perhaps err on the side of caution, but I wouldn't expect that you will need them, unless you look particularly shifty or sweat a lot when faced by people in uniform...