Have you heard of being in sleep debt? Sometimes (and of course this may not be the case) kids and teens keep taking a little off of how much sleep they need. The deficit slowly builds up until they need a lot, and oddly this can make them have trouble sleeping. I would try having them read before bed, use some lavendar lotion or oil, or even look at melatonin as a natural supplement.

Cedarwood and Lavender essential oil can do wonders.  Cedarwood especially makes me sleep very soundly.  I also agree that reading is a good idea to help wind down — no screens for at least an hour before bed.

I second the diffuser with lavender oil and have you ever tried a natural sleep aid for them?

Could it be stress, anxiety, or hormones? I know I am not a teen, but those things definitely affect my sleep. I've also heard people say certian vitamin deficiencies can affect sleep and that taking whatever it was they are lacking helped. Sorry I don't know which vitamin...

I'd say make sure they aren't drinking a lot of caffeine drinks, there's caffeine in almost all sodas regardless of what the label says. Also limit the amount of sugar they get especially around dinnertime. I'd also make sure they get sufficient exercise and outdoor time to help tire them out. If that doesn't help I would have them checked for any kind of health problems or deficiencies. My kids could both sleep thru bombs going off.

I began to notice in college that my lack of sleep caused my cognitive abilities to deteriorate very much. It became difficult for me to concentrate, I began to forget a lot, I was constantly sleepy and tired.  My doctor prescribed me smart tablets, these are http://nootropicboost.com/ . These pills helped cope with lack of sleep and its devastating effects. It seems to me that it's very important while you figure out the reason for insomnia so that they do not miss school and do not stop learning.

Anything that induces boredom will put them to sleepMaybe lego blocks or dominos or playing cards? 

Have you trid giving them some melatonin? It helped me when I was suffering from insomnia. It is prodiced by the human body as it is so I think that it is not as harmful as some other medications. I have periodic insomnia but when I have these problems they unfrtunetly do not come alone. They usually are acompanied by axiety and even panic attacks. I have consulted some doctors and aside from therapy they also suggested I try CBD. I agreed and since I do not like to smoke and oil is not very convenient to carry around I usually use edibles. I have tried different brands but right now I am using one that I ordered from this website: https://www.koalitymedicinals.com/. I have to say that I really like the brand. They have really nice products as well as really good shipping prices. I am not sure they will help you but it might be useful for you. I wish yuo will get the problem solved soon.