Floating and Buoyancy are key to learning to swim. With Back floating specifically being an important safety skill as well.
However developmentally, once kids start to sit up and become mobile (usually around 4-7 months) they no longer enjoy being on their backs as much.
This is where they may resist during lessons and become frustrated or upset – and it is completely normal until they are around 2 and they begin to understand a bit more and feel more comfortable with the skill – even then they often only want to do it on their terms.
Practicing back floating in the bath is a good way to start to encourage kids to enjoy back floating a little more. They are in a calm environment, in their home and in a small amount of water so they have more stability and control. Getting them to lie back and have their ears in the water while happy and relaxed can help when they are in the pool.
However while all floating attempts should be encouraged, it is important not to force them. Kids that are thrashing and crying will start to connect swimming with stress which is something we want to avoid. So remember to listen to your kids and only encourage them rather than force.
If you are finding your kids especially resistant talk to your teacher about different holds/techniques next time you are in class and remember to keep at it, they will get there eventually!