I have to disagree with the kids having ongoing exposure to their dad if he is the guilty party. If he is an upstanding guy that flipped out once due to excessive pressures, became excessively violent and injured Allison so badly she appeared dead - he'd have rung an ambulance and fronted the police, not put her in the water for almost 2 weeks and called a lawyer. Or confessed at any time since.
If he is, as has been suggested, a slimy, cheating, violent tempered man, prepared to stack a car rather than front police, god knows what he is saying to those girls behind closed doors to disparage their mother, sowing the seeds to make them feel like Allison was in some way responsible, make them not trust police or their grandparents, and potentially confuse their memories or opinions. If he can put Allison's body out in the elements to hopefully go unfound, he will surely be messing with their heads to protect himself. He won't suddenly only have their best interests at heart if he is prepared to put them through their mother missing, thinking god only knows what has happened.
If he is the one who murdered Allison and dumped her body, and he is at all concerned for his daughters above himself, he will summons up the courage to go straight to police, confess, plead guilty and serve his sentence without expensive representation and delays. He can do that at any time and put an end to the wait for Allison's family and friends and free up the resources of the police for the community's sake.