I personally LOVE x-acto blades . . . couldn't live without them. But, man, are they super sharp! The two accidents that I had were both because the blade slipped. It was fast, and I didn't quite know what had happened. The cuts were so clean, quick, and painless that there wasn't even any blood . . . until 30 seconds later when it came gushing out.
On my own models, I usually only provide a warning about blades. And, I know that kids way younger than 9/10 years old put my models together without a single problem—you can't put one together without a craft knife. Kids are definitely not dumb. I have two nephews (3 & 9) who are smart as whips.
When I spoke with Bruce Linder from the museum, safety and school policies were two issues. Not a big ones, but something to keep in mind. We both saw 4th graders using some type of blade (how else are you going to cut slots & slits?). So, we want to offer some details as being optional cut-outs. It's really up to the builder how much effort they want to put in. And, I'm going to try to do it in a way that will still offer a great-looking model even if those details are not cut out, so there's little/no penalty for skipping a few steps. It won't look like someone "forgot" to cut something out. But, the museum has last say, and everything goes to a committee. We'll see exactly what happens.
Oh, and there may not be round masts. The museum's model group had a hard time forming Mr. Detyna's masts, so they've asked for an alternative. I personally think that round masts are always the way to go. Since I'll be using thinner media, I'm going to have them try forming the masts again, along with some alternative versions. The alternatives will not look as nice as rounded masts, but, again, the call is up to the museum.
P.S. I LOATH round-tipped safety scissors!