Make loft bed taller?

Do you have a picture? I imagine the best bet is replacing the legs completely with pre-fab legs or calling the manufacturer and getting the tall legs instead. I wouldn't shim up the existing shorter ones. You never know what may be going on up there with guests (or their kids).

Oh, sigh, I thought that's what you'd all say. We'll probably just leave it as is (too short) then.

I agree, if you're not particularly handy, just get longer legs from the manufacturer. If you are a bit handy, cut wooden extenders to the same width and thickness as the original legs and whatever length you'd like to add on to them. Then attach them on to the ends of each leg with a http://www.woodworking-online.com/chapter6/2.html that is double the length of the extension. Make sure the scab is through-bolted in at least four places.

Well, you could get some bed risers. I'd get the wood ones, not the plastic ones. I am not sure how sturdy you'll find it, but you could try it, anyway.

The trivial solution is to put something under the legs. This would likely be impossible due to the conflicting needs for it to be safe, elevated, easily done, and appealing. Perhaps try calling in an experienced carpenter/handyman for a novel solution, if you're not against paying for that type of thing.

This sounds like a bad idea. Having a bed collapse is not fun. Especially a bunk bed. (Unless you're having sex on it at the time in which case it can make for a great story. If no one actually gets hurt which is very possible.) You can raise a normal bed 4-6 inches with those truncated pyramid bed raisers, but I would NOT recommend that for a bunk bed. Getting the longer legs from the manufacturer is probably the best approach. If that isn't possible I would dissassemble the bed, remove the leg, take it to a hardware store or someplace that can do a little basic carpentry and ask them to replicate it, but longer. Those are standard dimensional lumber sizes 4x4s and 2x6s. Or hire a handyman to do all this for you.

It depends just how averse to disassembling and reassembling you are. The legs are standard 2x6's and the cross-beams are 2x4's. If they're not too modified (with specific bolt holes or additional supporting cuts), it sounds like it may be feasible for you to just buy new 2x6's to your desired length, and attach those to use those as the legs. If that's not feasible, though, it sounds like you may be stuck with a slightly-shorter-than-desired bed.

I bought what is probably a similar kit, and the legs are not just stock 2x6's. Mind had notches cut in them for the 2x4 "ladder rungs" which contributes to stability, and of course bolt holes. They aren't tough to make, but you'd still have to disassemble and re-assemble the whole thing anyway.

Maybe this fits in with your aesthetics: landscape bricks are available in a 12x12 inch size; they're typically about 2 inches thick. http://www.menards.com/main/building-materials/landscaping-materials/patio-blocks/12-slate-top-patio-block/p-1321818-c-5785.htm. Stack four of those under each corner and you've got your 8" rise. They'll be stable and the legs aren't going to slip off them. In MN they're < $2 each. Maybe squirt some construction adhesive between them for good measure and put a thick towel or some low felt feet on the bottom to protect the floor.