Yes I can't see that any child would ask more "difficult" questions about the stickers than they would (say) about a tampon dispenser, or a lot of newspaper headlines. A parent might find it was a conversation they didn't want to have right there and then, but there's nothing offensive about them.
Can I reitierate the plea that people don't vary the message by mentioning "trans" at all? I feel diffident asking as I'm relatively new here but it seems to me that the "originators" of this have gone out of their way not to; and I sort of think that's important, because it means that when Penis News get all arsey, they simply look mad (to most people) because who mentioned trans anyway? Think of it like this: the stickers didn't say "A lot of TWs have penises". BUT PENIS NEWS DID SAY THAT, indirectly. I think that's sort of important.
I'm not saying no one should vary anything at all (am rather taken with the No Such Thing As A Woman's Willy direction, in a school playground sort of way!). Just that, it seems to me that the sort of hard hitting "branding" of the original stickers should be respected in this one respect.