Good ideas, guys, thanks. I've since realised I should be grateful that these combats are only lasting three rounds. School kids can take a looong time to finish their turn.
So by that rationale I've decided to keep pushing higher CR encounters at them and simply have a contingency plan if I TPK.
For example, Dragon of Icespire Peak has the PCs sallying around Phandalin, saving one resident after the other from various threats (caused by the titular dragon). When the PCs fight the dragon they'll be 3rd-level versus the CR6 dragon. In the event of a TPK I can have all the residents they've saved turn up to save them. A sort of angry mob that can get the PCs back on their feet without stealing the limelight. It might even seem like a reward for the PCs heroics so far.
