What do you use for ID at home? Your passport?
If you are only worried about it for getting in and out of the parks - you don't need ID unless there is a problem. You KTTK card gets you in but if there is a problem with the scan they might ask you for ID.
Here, drivers licenses are issued by the states and people who do not drive can go in and get a state ID card. Works exactly like a license (even looks similar) except for the driving thing. Could you look into something similar at home?
If not - bring the passport. I always have photo ID on me. If you are really worried about the passport - I'd make a photocopy of it and carry that. I always copy my license and insurance card and leave that in the safe. It won't work for buying alcohol (now, I'm in my mid 30s, look late 20s and have never been carded at WDW) but you'd have something on you "just in case".