The redcap: a monster with pretty neat fluff, a few interesting abilities, and absolutely terrible RHD. Fey hit dice are bad enough when they're on a skillmonkey: on a martial creature they ruin what could've been a decent race.
Redcaps have okay stats, with +2 or +4 across the board (except intelligence). DR 5/cold iron is neat. Eldritch Stone is mostly a way to give the redcap medium-equivalent slings, which given how terrible slings are still isn't great. At early levels it does have the benefit of giving you a way to hit past DR/magic so that's something. Powerful Build is surprising to see on a small character, but being counted as medium doesn't do a lot if all comparison points are going to be medium.
However, the RHD cripple the redcap to the point where a comparison to, say, an orc fighter becomes bad. The orc fighter is either hitting more accurately or harder, has equal HP, will get iterative attacks sooner, has a bunch of free bonus feats, and got better racial support to boot. The redcap does have an advantage in getting better DR and AC, but as always: being hard to kill isn't a great trait.
As always, something that's on pretty equal footing with a fighter is considered -0 LA. I suggest shaving off one or two RHD to make these guys more playable.
+0 LA when accounting for the advancement ability, poorly-written as it may be. It gives quite some increased power over time and imo makes redcaps okay high-level picks. Kinda.
Minor differences with the normal redcap and a bunch more RHD. -0 LA.