Build one. I've built all our beds, and we currently have the four youngest sleeping in one room. (They sleep better and bedtime is happier with four books read all in one room.) It is spectacularly easy to build a bunk that is bolted into a wall. If you build it yourself, you can adjust heights and railings as needed. Your children are probably too young for anything that is on the standard market. However, it would be simple to custom build something that is reasonably safe for them, and sturdier than anything you could buy. If you have the hardware store do all the cutting, you can have it up in less time than it would take to buy and assemble one from the store.
The costs are a couple hundred dollars and a couple hours of your time. This is assuming, of course, that you are lazy (like me) and don't bother to put a fine finish on them. If you want them beautifully finished, that will increase the time significantly. As far as I'm concerned, though, unfinished plywood will hold up a mattress just fine!
Check the safety standards for appropriate spacings to avoid head entrapment. However, the biggest risk with bunk beds is children getting trapped between the wall and the back of the bunk. Since my bunks are structurally part of the wall, this is not possible. A pillbug couldn't get between the bunk and the wall, much less a child!
Look up how to design one of these at the library, but here's the basic concept. I use the fancy decking screws that zip in easily and have a shear strength of something like 2400 pounds. A power drill speeds things up immensely. I don't do any fancy angle joints. Butt-joints work fine.
1. You make a frame out of 2x4 lumber that is the same width as a mattress. You can make it longer if you like, for extra storage on the bunk.
2. Bolt this frame into a corner to the wall at whatever height you like. I bolt into every stud, so it's non-catastrophic if I miss a stud. The frame will be supported by the wall on two sides.
3. Add a leg on the single corner that is not touching a wall. I usually use a 2x6 as the leg.
4. Put a piece of 3/4 inch plywood on top of appropriate size.
5. Add a railing. Make sure you follow safety standards regarding spacing. You can make railings out of 2x4 lumber or 1x2 lumber, depending on whether your kids are likely to play Tarzan on them. The uprights work best as 2x2 lumber.
6. Add a ladder. I usually buy a wooden ladder from the hardware store, cut it off at the appropriate height, and attach it. Sometimes you can scavenge a ladder from another bunk bed. You can also build a ladder into the bunk itself. Check spacing to avoid head entrapment!
7. Put a mattress on the top and have a party!
Good luck!