uh, not unless you're trying to be funny...?

boylet? girlet? ;) No, that doesn't work too well in English. There are, of course, the Chippettes but that has nothing to do with them being "small" or "cute" - but merely to signify that they are the "ette" (ie, female) version of the "Chipmunks"

And saying "little girl" instead of just "girl" or "little boy" in place of "boy" isn't the same as using "chan" or "kun" either, as chan and kun act as an actual extension of the name. I suppose you might be able to say "little Kero" ... but it still doesn't have the same meaning, really.

Once you've used -chan and -kun as often as I have, and after having heard it used as often as I have, you'll start to understand better what it actually means... but it's a little hard to describe to people unaccustomed with Japanese culture.