Do you have any access at all to a trainer, or someone with more horse experience? 2 1/2 and 5 1/2 are both young - you're essentially dealing with a 5 year old and a 12 year old on the maturity scale. As a novice, you'll need some on-site guidance, especially with such relatively young horses, and by the sound of it sooner rather than later.
Kicking cannot be tolerated, and your window to make this known to your filly is about 3 seconds - longer than that and she won't associate the correction with her infraction. Whether she was "just testing" or being more serious in her behavior, the rule still holds.
If my horse had kicked at me, he would have thought that I'd gone insane... drive the horse out of my space as fast as I can get those feet to move. If it takes a smack it takes a smack, with my hand, a brush, whatever. It will not hurt the horse - compared to another horse exercising the same authority it's nothing at all. That being said, it takes timing and no hesitation to be effective, something that, as a novice, you may not be able to accomplish right away.
Alongside recruiting some on-site help (either a trainer, instructor, or horsey friend), keep researching - your filly probably gave you some advance warning that she was thinking about kicking. Look for pinned ears, stiffness, no blinking, clamped tail, etc., and take that as your warning and nip it in the bud before she escalates to kicking.
Also, make sure that your family understands the concept of "the danger zone" and exercises extreme caution around both of their rear ends - either come behind them close enough that you're rubbing their bum (too close for a kick to have much force behind it) or 10 feet away (too far to reach).
Best of luck!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown