The way I see it is this: Your daughter in law's share of the house is left absolutely to the children (there may have been a life interest though - you need to check this with her will). As such, this has never been owned by your son and thus is outside of his estate for divorce purposes. He only owns 1/2 of the house.
Say they divorced or your son predeceased - it is impossible to say whether the court would enforce an order for sale to get the new wife and children out of the house. You would need to make an application under ToLATA 1996 (Trusts of Land and Appointment of Trustees Act) for an order for sale. THe rights of infant beneficiaries will always be accorded preference by the court. However, this also has to be balanced with the "majority rule". I cannot see that your son's new wife and children can bring themselves under the "original purposes of the trust", so I see you being able to enforce a sale slightly more likely. However, there are many factors to be taken into account at the time.