so as a DM for my kids, we finished the 1st adventure in the redbox (from 4e but made it work with 5e) and at the end they actually convinced the rogue to work for them and join their group.

so now we have him as a sidekick (thank you new essentials kit) so our Level 3 Human “expert” named Eugene Crabs, has reached level 3. They beat the goblins outside of Cragmaw Cave, I’m leveling up the sidekick (Mr Crabs) and was looking at HEX for one of his 1st level spells.

question being, can Sidekick Crabs use his bonus action from “cunning action” to cast hex and do a Hex + Sneak attack Combo for a total of 3d6+ his 1d6+2 shortsword damage?

Well, first off, the expert doesn't learn spells.

Second, you can only use cunning action to do what it says. Namely: dodge, dash, or hide.

Third, hex is a bonus action spell and is always cast with bonus action.

Okay cool I kinda figured since the rule on it was so specific, also the “expert” thing kinda threw me off as it wasn’t a class that I was familiar with, I’m assuming it’s a kind of generic thing for sidekicks?

Yeah, the sidekicks don't really have classes so much as roles to play and are given features to play that role.

You can make changes to them if you want (homebrew etc).

Strictly speaking, experts don't even get sneak attack, though they do eventually get extra attack.

I believe that your side kick has a nemesis named Plankton and can give a boon of employee of the month.