My first impression is that this is a particularly stupid idea. It will probably result in lot of kids not seeing the number because they're a little too short or a little too tall for their age.
For two, they're explicitly taking the 'average height of a 10 year old' for this. You would write off the entire ad as pointless because, as implied by the very definition of "average", 50% of 10 year olds and a smaller percentage of each age below 10 would be unable to see it?
Talk about throwing the baby out with the bathwater...
According to what I find from a quick google, average height for a 10-year-old is about 4.5", with it starting to spike up dramatically at the age of 12-13 (from ~50 to ~60"). Without knowing the actual height they set this for, though, it's impossible to say how many kids they would or would not get with this kind of advertising.
Interesting idea, I guess it's so the abusing adult doesn't hastily pull the kid away from memorizing the number?