I won't claim to be an expert on all things scouting, but I've been involved in scouts at one level or another for many years now, as a den leader, cub master, assistant scout master, and merit badge counselor. Here's what I know or recall: There are very few rules at the national level regarding knives. Fixed blades are allowed but not encouraged. Many sources claim a 4 inch max blade length, but I'm unaware of an official policy on it. Same goes for locking blades: there is no rule regarding requiring a locking blade. Many scouts carry slip joints, and in fact most "official" BSA knives sold at scout shops are slip joints. (I would not recommend most of those knives though...you don't know who the manufacturer is, and while they are all stainless, most don't even tell you what kind of steel. There are plenty of knives from reputable manufacturers that also bear the official BSA badge if that is something you are looking for. Find a copy of Boy's Life or Scouting magazine and look in the ads in the back.)
I personally think a slip joint is the safest. We are all adults, at least age wise
and are (hopefully) adept at manipulating a liner or frame lock. A ten year old, maybe not so much, and the lock is encouraging them to put their fingers in front of a blade that is about to be moving. They may not have the experience or the dexterity to close the blade in a controlled manner and/or clear their fingers from the blade path. I have personally witnessed boys having issues with liner locks, which I think are harder to manipulate with smaller fingers and less muscle and dexterity, though I haven't seen anyone cut from trying to close a knife...just come close. (Using my logic, I suppose back locks would be ok.)
A Cub Scout is required to have their Whittlin' Chip in order to carry a knife. It is a wallet card, and must be on their person any time they carry. They can also get a patch for their uniform, but I think the card is still required as it is signed by an adult leader. A Boy Scout must have earned and have on their person a Totin' Chip. If the boys are carrying and asked to present their chip and don't have it they have to give up their knife for the event/meeting/camp/what have you. I've never seen a boy asked for it unless they were abusing the tool or violating the rules. I think a cub scout has to be a first year Webelo (fourth grade) to earn their Whittlin' Chip. Someone could certainly give a boy a knife at a younger age, but they won't be allowed to carry it at scouting events.
Individual units/packs/troops may have their own rules. I don't know if your requirement for the lock is your preference, the parent's preference, or a rule of the unit, but by all means abide by that as opposed to my opinion. But I will encourage whomever is suggesting the lock to evaluate the advantages vs. disadvantages.
Having said all that, I think a SAK is the way to go. I think the Farmer and Huntsman are great choices. I've never seen a boy that wasn't fascinated by the multitude of tools, and it is a classic and instantly recognizable. The boys just respect a SAK because it has an image/reputation. Get him a nice little sheath with a snap (not velcro, he'll lose the knife, trust me) and he's good to go. And those extra tools will one day come in handy when they are camping and realize they did indeed forget a can opener
