I know some people will have other methods. But here is what has worked for me for many years:
1. have them use their finger to wiggle the skin under their bottom lip. Then, ask them to use their facial muscles to stop the finger. ask them to do this many times to memorize the feel. It also helps if they see it when it is correct.
2. If your kids have patience, you can discuss vowel shapes for low vs "high" notes, while also addressing that air speed will have to increase. Think of blowing farther and faster. There are plenty of air flow activities you can find online for this. I adapt Breathing Gym for my beginners.
3. I am a firm believer in lip buzzing, or "free buzzing". It has given me great results with my students. Sometimes for the strugglers, I ask them to wet their lips, make "the face" and gradually bring their lips a tiny bit closer together. Sometimes they can cheat a little bit by using their finger to pull their lower lip down, and then once they get the feel, then they dont need it.
Having said ALL THAT, sometimes some kids struggle, but as long as I can get them to play those lower notes with a big fat sound, then I am okay with that. They will probably catch up eventually. Good luck