I would tell her ... holding the information back can be interpreted as lying. Moving is a tough time for our kids -- but I think the letters will ADD to security rather than detract from it.
I think you know our story. Bfamily contact has been VERY healing for the boys -- for much of the same reasons you mention. They worried -- knowing I know they are ok and nothing bad happened to me by knowing them has been a good thing.
I would also start another thread about moving

Our last move was EASY ... and I think we did some things differently which helped. Maybe others have ideas as well?
Jen