As much as I'd say replacing the belts would be a good idea regardless of who rides in it...
I'll admit I had a 1979 Pontiac Grand LeMans until DD was about three years old (2009) and never thought to replace the belts. At that point it was my "summer" car and when she was FF she was FF in the front passenger seat, tethered to the rear seat seatbelt behind.
It would probably make most sense to just switch cars if he needs to transport the kids, or if all five of you go somewhere together.
How old/big are the kids? If he needs to transport the kids only (without you, just four people) and for whatever reason (emergency?) he can't take your car could he have each child in a booster with a lap and shoulder belt, in the two rear outboard seats and the front passenger seat? You should check to be sure but I'm almost positive the car wouldn't have a passenger airbag. HBB in the back seat at least as the wouldn't have headrests.
Edit: Even my 1979 had a long section on child seats. It mentioned locations to drill anchors, as well as the safest location for a child to stand if there were more people than seat belts! Thankfully times have changed.