get it fixed for less than they pay you.

The thing is, the insurance company will make you think that you can just take it to one of their approved repair shops and not pay a penny and how convenient it will all be. You should get a couple of high bids for the repair yourself if your son can. You're entitled to a check for the damage instead of repairs. Get the check.

If you can live with the crack then cool. Money is yours.
If it was me, and the crack was cosmetic, I wouldn't fix it on a bumper. Odds are the bumper will get hit again, or the car will get hit again, or after 4 more years you won't care. You can always use the money to freshen up the car before you sell it if that's the biggest issue with it.