Merry, Pippin and Gimli's kids would be the naughty type.
They would sit around in a circle in the basement and smoke pipe-weed (true
That 70's Show style) and convince little Legolas Jr to get them into the wine-cellar, because he is the only one of the offspring anyone trusts.
Faramir's kids would be pyromaniacs because it skips a generation, and because of Eowyn's energetic DNA. But they would also be shy. Quite an odd mix.
Morgoth's child would catch bugs and corrupt them. Where do you think mosquitos come from?
I can imagine Sauron's kid being able to change forms, but he would end up as a little everything: a werewolf puppy, a vampire fledgling, a little fiery eye...
Little Krima Wormtongue would throw things, valuable or not.