My suggestion is slightly over your 2.5 hour journey limit but it might still be worth considering:
If you catch the 0710 train from Hereford (on Saturday 19 January, 6 days after your son's birthday) you can be in London at 1029 (or the 0617 train gets you there at 0929). The 1821 train back gets you to Hereford at 2135. Those are direct services, with no changes. The total return fare, for 2 adults and 5 children, is £105 including unlimited travel on London Underground and bus services.
That would let you explore the wonderful Science Museum, especially 'LaunchPad' (which is packed with things to do and buttons to press) and pay a visit to the moving dinosaur at the Natural History Museum next door. Both museums are free but your budget would also allow you to pay for the few charged exhibits (such as the IMAX cinema at the Science Museum). With careful planning you could also squeeze in a bit of sightseeing.
(I've just checked: You could also use the same services, at the same fare, on the day before your son's birthday but Sunday services don't run early enough for a trip on the big day itself).