I've been singing the kids Nat King Cole's L-O-V-E for ages, and they're both super into it! Probably because it involves the alphabet:

Lately I've been singing You are the Sunshine of My Life to the kids... no spelling in it, but I think the kids are getting into it too!

Any other oldies that the kids might be into? Anything involving spelling is a win... or anything about love seems to work, because we can sing to the kids about how much we love them, and they love that.