Football cards have always been my favorite even though I collect all sports.
As far as getting going in collecting the sets I think picking an era first is the start.
Junk era cards are plentiful and a cheap way to start but the sets are fairly big. If your boys are more into modern you can pretty cheaply collect non-rookie base cards as a start.
I just picked a few examples:
2021 Panini Prestige 1-200 are veterans, rookies are 201-300. If you focus on the veterans that's a 200 card set.
Most of the Panini sets are this way. I think collecting the veteran portion of the sets in current is a nice way to get your kids into collecting and has all the big names, just not the expensive rookies.