In reply to The Lemming:
Some thoughts to add to previous advice:
Yes, definitely Farne Islands boat trip from Seahouses. Alnwick Castle has lots on for kids in summer holidays - knights, Harry Potter etc.
http://www.alnwickcastle.com/whats-on
Depends on age of small Lemming, Pot-a-Doodle Doo in Scremerston keeps kids happy for an hour.
http://potadoodledo.com/
Bird of Prey Centre at The Barn at Beal on the road to Holy Island.
Great beach walks at Scremerston, Chiswick Sands, Holy Island, Bamburgh, Beadnell, Low Newton (to include the Ship Inn with its own micro brewery)
Bouldering at Bowden is too tempting to miss!.
The fleshpots of Berwick, plus a walk around the City Walls
Chillingham Wild Cattle and Castle, with its weird and gruesome Chamber of Horrors - but not in the best possible taste IMHO
Heathersaw Mill Light Railway near Ford.
Animal Park at Eshott on A1 just south of Felton.
Hope this helps and that you have fun.
Dave