Does not matter what level of strength you use, but if you have the strength effect, you can kill medium and large magma cubes, but not small magma cubes. This does not affect slimes.

Steps to reproduce:

1. Run the command `/effect @s strength 999 1`
2. Spawn a magma cube, and reduce it to small ones.
You cannot kill the babies by clicking on them. If you run `/effect @s clear` you can kill them again.