The game play itself would be fine for my almost-5-yr-old, but the in-game ads may be bit inappropriate at times: "Daddy, why is that lady fighting in her underwear?" in response to a fantasy game ad that was early in the game -- an ad featuring a buxom woman in a corset brandishing a sword. I'm hardly prudish, but some of the fantasy game ads are a little overly-sensual, and might at least raise some eyebrows, not to mention that they might introduce the concept of hypersexualization of women at an early age, if that is of note to you.
And though you have to press a button to view the ads themselves, that button is right in the middle of the screen, and a bit more tempting to press than the "skip" option -- and that "option" pops up each time Sonic crashes.
I'm not opposed to ads in general -- but it would be nice to at least be able to set some parameters on the content of the ads. Or have a paywall that would allow you to get rid of them altogether -- I'd gladly pay a couple bucks to circumvent that.