imo an ar15 is a terrible rifle to start a 5 year old with. get her a gun she can feel comfortable with. a small 22rf of some sort for starters.
other than being noisy, they're low recoil and the adjustable stocks make them much easier to shoot for small bodies.
buddy who hunts with us brought his daughters (6 and 9 at the time) up to my parents farm to learn to shoot. the 22lr was just to long for them (he didnt have a cricket) and even the older kiddo was struggling.
got out my 223, put the can on and closed the stock way up. she (the 9yr old) was popping balloons in no time. after a couple mags suddenly she started to "miss" again. several shots in a row.
we asked if everything was ok and she said "yea, i'm not aiming at the balloons (35 yds) i'm using the big target down where you and dad were shooting (100 yds)". little monster gave us a big old sh!t eating grin and put her head back down on the stock to finish off what turned out to be about a 4" group.... all this maybe 100 rounds after pulling the trigger for the very first time earlier that afternoon.
both my cousins daughters (now 13 & 15) had shot my ar before they were 6. the older one at like 4 yrs old. both loved them - same reason, the adjustable stock made it comfortable, and the scope made hitting stuff easy.
YMMV of course, but i've seen nothing but positive with them.