My DS2 had butter popcorn poops for the longest time. Both DH and I both smelled it, so it wasn't just my odd sense of smell. We used sposies and CD, so that was not an issue.
DS's poo smelled like buttered popcorn for a good 9 months of his life, but it is gone now. I have no idea what caused it, but he wasn't ever ill so it wasn't caused by anything serious.
Maybe your DC is eating something that causes their pee to smell that way. Certain food make my urine smell funny. Just a thought.
Mom to DS1, DS2, DD1 and DD2!