Will the youngest still be under 3 ?
If so they do not count towards occupancy, so you would only need a standard room.
There are five of us and every visit we book, we look at all the options but the supplements for the family rooms at DLH can be £300 per night so we have preferred to stay at the Marriott as you get a 2 bed house with 2.5 bathrooms
However the DLH is a fantastic hotel and definitely worth staying at (we persuaded the in laws to go and officially one of our children was in their room)