Our daughter has asked for a DSLR camera. I’m proud of her and the reason that she wants it, but fairly horrified that she wants such an expensive gift.
The canon 450d is about 10 years old, but is still a decent 18mpixel camera that uses SD cards, and is about $100 - $200 on ebay without lenses. You can go older, but the models before then use compactflash, and you'd probably want to get a compactflash-->SD adaptor (that's what I use on my old 400d).
Of course, then you're just lending your expensive lenses out, so get a package with two old starter lenses so she has little reason to borrow yours and see what happens. If she's careful with the cheap one, you could lend the expensive one from time to time if she has good reason, such as shooting video.
These cameras don't seem to lose much value over time. I'm surprised that the camera I bought over ten years ago for $650 still commands over $100 as a used item. But that also means that if it doesn't work out and she gets bored with it, it'll still have some resale value a few years from now.
It should also help her understand that the difference between an inexpensive camera and an expensive one is not a big difference. Big enough that it can be worth it, but you can get great images and learn just as much from a low end model as a high end one.
That may still be an expensive gift (it certainly would be in my family!) but it's probably the way to go vs getting a body incompatible with your current lenses, and you may find times when you'd like to take a camera out but don't want to risk your nicer one, so it could benefit you as well.