The BG Flex Series was a technique saver back when I was playing several hours a day and I went through thumb fatigue.
Just make sure they kids aren't pushing their heads forward to meet the mouthpiece. The neck strap should be tight enough to hold the mouthpiece at their mouth with a little guidance from the right thumb. And the right thumb should still be involved with slightly pushing up. The neck strap is only there to relieve some of the weight of the clarinet.