Here's the secret:
Thin coat of peanut butter on one slice of bread.
Thin coat on the OTHER slice.
NOW put the jelly (I'd recommend jam because it's thicker and more consistent to spread) IN BETWEEN.
The peanut butter protects the bread from getting soggy.
I know, brilliant, right? And I'll even let you take the credit with your kids for being the smartest mom ever. :)