Small person has a Cannondale Trail 24" wheel bike (I dont recommend it). It's got a 1x8 drivetrain. In the highest cog the chain is a bit too slack and occasionally jumps off. I'm going to try shortening the chain by a link. But I suspect this would make the chain too short for the lowest cog.
The rear mech is a quite old Acera that might be a bit worn so I'm debating changing the mech. I can't find an 8 speed mech with a clutch so looking at Alivio. This appears to have a much longer cage. Would a longer cage solve the problem or would it make it worse?
TIA.
Longer cage allows for a wider range. The wheel size makes no difference to setting chain length (of course it does but only by the chainstay length, the distance around chainring and cogs is the same whether your chainstays are 18" or 18ft)
What range is the cassette? 8 speed I'd guess at not more than 11-30 or 11-32, so surprised it would need a long cage on a single ring.
Photo?
Sounds like a worn chain and cassette to me to be honest. Was it all working ok when new? A fresh 8sp cassette and chain should be v cheap.
The bike is brand new, except the rear mech. It came with the very cheapest microshift rear mech which was so floppy I swapped it for the Acera one. (Along with all the contact points as they were too big for me let alone a small child).
The freehub was a bit sticky but I've stripped it down and replaced the treacle with some of Peaty's race grease which has improved things a little
Ok, got a better understanding of where you are now.
it’s probably a matter of working through the standard setup now really. Yes, check chain length, that’s one tick box
https://www.parktool.com/en-int/blog/repair-help/chain-length-sizing
in this case though with a 1x8 setup (kids bike setup can fall out of most internet guides) I’d be looking to get the chain as short as possible whilst still fitting in the biggest cogs front and back through the derailleur.
however, slipping on cogs isn’t usually a chain length issue. Check b-tension would be next on my thoughts, especially as you’ve replaced the mech.