I have recently went thru this with a house on my road, people bought house, move in, trash house, get the big home buyers check from the .gov, along with the tax breaks for 4 kids.
There was a residency requirement, I think it was 3 years, to keep the home buyers tax credit without having to repay it. The IRS will start keeping their tax returns (which will probably be substantial with 4 kids and EIC) to recover the tax credit amount.