If you have no status, the way to do it is to pay, I'm afraid.

BA will ensure that you and your kids sit together, but not your husband. If you want to avoid paying for 4 assignments, you could wait until 3 days before the flight when you should see your 3 seats and hope there's a spare seat next to you which your husband could then pay for, but that's absolutely not guaranteed.

If you really do want to make sure all 4 of you sit together, you should really pay now.