I have two young students whose 5th finger tends to buckle if they play loud. Their 5th finger seems to have almost no strength at all and they have to rely on manipulating their hand position to play the 5th finger (leaning to the side) which is quite worrying for long term development. What do fellow teachers have to say about fixing this problem? I don't remember having this problem when I was learning, so I'm not too sure how to best guide them on this.
Practise bringing the thumb over to touch the fifth after sounding. This will open and strengthen the hand into a proper position. Then, keeping the hand open, point the thumb straight down instead. The most likely cause is raising the thumb, which weakens the knuckles and lets them droop. Also practise playing with the fingers in a wide open arch while touching the thumb against the wood below the the keys. It stops the droop creating weakness.