They may be on the wrong instrument. It sounds like they would be amazing baritone players instead.

The fact that you have two tells me that they are reinforcing each other's dysfunction. I think you are more likely to solve this problem dealing with them as individuals rather than a duet.

Roll lips in.

Have them blow through coffee stirrers without biting down on it. Then slip the mouthpiece over the stirrer and have them continue.

You are probably already saying what you need to be saying, some kids just take time to get it together.

Fast air
Small fast air
Cold air
Aim your air down
Smaller aperture

Sometimes I can coax a higher buzz by having them buzz on their mouthpiece and slide from low to high, then ask them to start on the high buzz they just ended on.

Double check and make sure they aren't doing anything weird with their embouchure- a lot of times my kids that struggle getting to pitch are pushing their lips outwards (duck face) or allowing the shiny inside part of their lips to come out around their corners. These will need to be addressed if they are going to make it. Some kids eventually fix it- some kids do not.

I will get some disagreement here, but sometimes I'll tell a kid to "buzz as high as you physically can" or "firm up your lips" and ignore what would be considered good pedagogy to get a kid over a hump like this. This sometimes forces some good changes and it is often easier for them to work towards making a good sound on the note once they have experienced a little bit of success.

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