If you feel the 9 yr old will obsess over it, then I wouldn't give the cards to the kids now. Especially as you say, school is about to start and they don't need to have that on their minds right now.
I'm just wondering what Grandma was thinking in writing something like that to a 9 yr old. I don't know the specifics of your situation, but I can't see any 9 yr old reading that and feeling like Grandma really loved him. You know how kids are, they think that everything that happens, happens because of something they did, so words like that could lead to some problems for him. If that one line wasn't in there, it would be fine.
PS- do you have a big black marker or some whiteout???
