[BizarroLand]

My guess is that a lawsuit would follow and whomever messed up would be on the hook for any losses between what the final divorce settlement was and what it most likely would have been had it been properly negotiated, plus any emotional damages coming from misapplied parenting rights agreements and the like.

Given that they were using online lawyers, likely that is not very much.

If this was in America, depending on the couple's total wealth and circumstances it could run into the several hundred thousand / low millions.

I don't know the laws in the UK or how that would play out there though.