Family tours of the U.K.
Tours aimed at families
Of course, a family can join most general tours, but—especially with younger children—you really want a tour that is going to cater specifically to families.
They will move at an appropriate pace, and be sure the sights and activities are fun for all and not just intriguing for adults and an unending cultural deluge for the kids.
(Also, none of the other participants will throw you nasty looks for bringing boisterous youngsters on "their" tour.)
The only drawback: There are precious few family tours to the U.K. (most offerings are to more exotic locales, or at least the Mediterranean).
You can check the family tour operators below to see if they currently have any British trips on offer—or just take your trip on your own and sign up for guided family-themed daytrips and activities while you are there (I've listed a bunch below).