I'm in the same boat - US citizen living in Canada. You have to present proof of vaccination OR a test taken 1 day in advance but since it can be a rapid antigen test, I'm just going to pay the $25 (there's a local pharmacy that does them for that price) and take it because I don't want that to be the thing that prevents me from getting on that plane!!
So for unvaccinated kids (we have 2!) here's what we've determined from the info on the CDC website (we spent ALL day yesterday reading up on this, my husband has printed off stacks of stuff from the CDC website and highlighted it all so we don't get caught with an agent who has the rules slightly wrong!) - because they are traveling with 2 fully vaccinated parents they do not have to quarantine for 7 days upon arrival BUT they (or the parents) have to attest that they will take a test 3-5 days after arrival. For this we ended up deciding to purchase a rapid antigen kit from Switch health that we are taking along with us so we don't have to waste vacation time traveling to an appointment. This is in addition to the test (either PCR or antigen) 3 days before arrival. Note that there is some wording that says unvaccinated travelers need to test 1 day before travel BUT there's another section that says if kids are traveling with 2 fully vaccinated parents they can test 3 days before travel. So if your kids are unvaccinated, that's 5 tests to take the trip to Hawaii!

(2 to get there, 3 to come back). This stinks for my kids and they're not thrilled about it but we're going with my entire family who are all American and traveling from Michigan so we are the only ones who have to deal with this hassle. If it was my choice I would postpone the trip until the kids were vaccinated!
Read the CDC website very carefully because it's super confusing and you have to go to at least 3 different pages to find all this info!! Hope that helps!