The Family ticket has been around for a while and is valid for a minimum of 1 Adult and 1 Child up to 2 Adults and 4 Children, with both Standard and First Class options being available.
Has to be booked by 23:59 the day before travel, with the outward journey being restricted to the booked train. For the return, you can opt for a specific train or, for a slight premium, a return that's open for a month with no time restrictions.
Quotas appear to be vanishingly small at present, however, as I just looked at a midweek Manchester to Euston journey for later this month and was only getting normal Advances and Off-Peak Returns.