We've been in the parks many, many times, with 4 kids (sometimes 3, before the youngest was born). We've never even come CLOSE to losing one of them.
The kids you have listed in your siggie...will they all be going on this trip? If so, the 1 and 3 year olds will likely be in a stroller, yes? If you're really worried (or if any of the kids in your group are prone to running off), get one of those slinky-cord things another poster suggested, and just keep any kid(s) who won't be in a stroller connected to either you, their Dad, or the stroller at all times when walking.
Also, if you're really worried, get each of your kids matching T-shirts in a bright color for each day of the trip, so that you can spot them easier.
My personal opinion is that it's paramount to keep hold of all very young children in some fashion at all times in a place like Disneyland. In your case, 2 in a stroller and 1 attached by either a slinky-cord thing or by simply holding her hand at all times, would do the job.
If a kid 5 or under has been unattended to such a degree that they've been able to run even ten feet away in a super busy venue like Disneyland, then the adults in the group should not be taking them to places like that. It's not just about abductions, etc. Kids that small could EASILY be seriously injured if they ran into the exit of a ride in Fantasyland or something. There are also places in the park where a kid could drown, or suffer a serious fall. If two parents can't, for whatever reason, physically keep hold of all of the very young kids in their group in some manner, then maybe Disneyland is a destination better left for when the kids are older.