I'm not sure if you're looking for "kid decals" or for something like adhesive squares that can be applied just to each tile to just change the color of the tile.

My sister-in-law sent me this link some time ago, I can't vouch for their product as I've never used it. I'm not sure if this is even what you're looking for, these are more kid-decorating stickers.

Kid decals

you're right. I should have been more clear, I definitely don't want kid themes!

I just want something classic that will disguise the old dated tiles. It will look kid-like enough with all the bath toys etc....

Have you considered painting the accent tiles? Below is a link to a thread concerning doing that.

Here is a link that might be useful: contractortalk thread on painting tile backsplash

I have actually painted some tiles in an old apartment. It was a toxic procedure, but we were painting the entire tub tile.

I was really hoping that there was something out there that was a whole lot easier and not with as many fumes!