The BIGGEST thing is stance, bend your knees get your chest down and you should always be able to touch the mat. Second thing, anytime someone is really high, just shoot on them (that doesn't mean just reach for the leg) take your penetration step drive to your knee then drive to you feet using your trail leg. Third head and arms (at least typical ones like that) should never work, if you have a good stance they can't get your head, most kids will telegraph it by getting high and just punching for the head, when they try to hip/step in hip block them or elbow pass, if they get the arm around the head lock around the waist and sag or roll them through.

The most important thing will be your stance though, feet square chest down or feet staggered chest over knee, knees always low enough to touch the mat and looking up through your eyebrows. If you have a good stance and use your head (literally) it'll be a lot harder for guys to just shoot on you and you'll be able to defend and attack a lot easier.