I have a group of new level 5s and they all have their kips but they are all doing ugly muscle up bent arm kips. I do have drills that I do with them but I am posting to hopefully get some more tips or drills on how to perfect their kips!
The fact that they are muscling the kip means they are probably kipping early. If they were merely bending their arms but waiting on the kip then they are just not strong enough or don't understand the lockout of the arms.
In my experience, the bent arms happen when the gymnast kips too late as the result of extension of a glide which is not proportionate with the abdominal/hip flexor/shoulder strength of the athlete.
While younger athletes may be perfoming "optimal" glides, a full extension in the glide means difficulty in lifting the feet to the bar. The glide tends to have a forward/backward motion instead of a rounded circling swing upward upon completion. If the athlete is not strong enough to lift the legs out of a fully extended glide, the leg lift occurs too late, the kip happens on the back swing of the glide, and the shoulders swing back horizontally behind the bar so far that bending the arms is the only way to "save the kip". The arm bend is not simply a form break, it is a mechanism to correct for a lack of circular momentum as the kip finishes.
While extension on the glide is important, I have found that a rounded body shape and an early lift of the feet is critical for beginner athletes. I tell the girls to stretch and lift their feet toward the high bar instead of out or down. The leg lift is part of the glide, not something that happens when the glide is finished.
I also have kids do millions of straight arm "jump uprises" from a block. The block is slightly under the bar opposite where the gymnast will finish in a support. This creates the circling motion which should occur at the end of the kip.
This has always worked for me with younger kids. Of course, the kip has to develop and change over time...