There is nothing in Catholic modesty that requires calf-length or longer skirts on girls. This is your personal preference and between you and your daughters. But do not suggest those draconian dress standards are required of a Catholic practicing modesty!
First off, “draconian” is an insulting term. I’ll thank you for using a tad more respect with regard to “my preference.”
Secondly, anyone who thinks that little girls can be modest in knee length skirts must not be familiar with little girls. Not because visibility of the knees is immodest, but because little girls sit on the floor “indian style” (or criss cross applesauce, or pretzel style), and because little girls play on the swings at recess, and because little girls run around where the wind blows. So a knee length skirt is very likely to actually end up showing skin very close to the underpants if a little girl in school is wearing it, if not actually showing the underpants.
Skorts and shorts protect against that kind of innocent flashing that happens all to often with little girls. So would bicycle shorts under a skirt, if they are long enough. But if little girls are going to be required to wear skirts, and bike shorts are not part of the uniform, then only a skirt that goes below the knees will keep from sliding right up to the crotch when that little girl is sitting on the floor for “circle time” or at all those other times when she is not being quite prim and proper.
So we could go back to the days when little girls were expected to always sit properly, and not allowed to run around like the boys. OR we could make the girls actually wear long enough skirts so that they can comfortably act like kids and sit on the floor at assemblies, or sit in “circle time” etc… without worrying about exposing themselves. Personally, I’d pick the one that actually lets girls play.
***Note I did make a caveat for bike shorts under skirts. I think that is a big improvement. But I also would like to point out, that even hidden bike shorts doesn’t really solve the modesty problem as well as it could. Until your skirt has actually flapped up in the wind and shown off the bike shorts, you are creating the impression of a potentially “exposing” situation. The anticipation of inappropriate exposure can make people uncomfortable (or titilate perverts or hormonal boys), and I think that bike shorts do their best work at enabling modesty when they fall just a bit below the bottom of the skirt (or if the skirt is long, when they fall below the knees). This way, the person wearing them assures those around her that she won’t be flashing them without having to actually prove it. I know, I’ve thought A LOT about this. But I’ve seen a lot of girls play on my swingset, and have had moments of nervousness too many times (worrying about them exposing themselves out in the yard) that I’ve decided that I’d rather my girls just wear pants when they’re playing like that.