I don't know why it is so endearing when preschoolers get their words mixed up, but it is. Right now my 4 year old daughter confuses "eagle" with "beagle". This is especially confusing when she's playing with a little plastic vulture from a Happy Meal, exclaiming "Look at my flying beagle!" (whose favorite food is apparently string cheese).
Other recent word switches that took a few minutes to figure out were "caboose" instead of "cafeteria" or "lobby". Why yes, we can go into the caboose at your brother's school.