Usually romance in kids cartoons is lame and goes nowhere (Ash/Misty from Pokemon), but then sometimes they try to rebound and do it right (Ash/Serena), even if that may go nowhere too.
I don't see how Ash and Serena is an example of trying to rebound and doing romance right, especially when it won't go anywhere either. There's no doubt about that. I can guarantee you that Ash will still be oblivious/clueless about Serena's feelings by the end of XY.
Anyway, having romance in a kids' show certainly isn't a bad idea. It just really comes down to execution. Legend of Korra is a perfect example as to not handle romance in a series in my opinion. The romance subplots were taking up way too much focus during the first two seasons, Mako and Korra didn't have much chemistry and their relationship was just painfully bad to watch with their constant arguing during season two. It felt forced and contrived to put them together, but at least they never had them get back together afterwards, so that was an improvement. Still, the romance for that series was easily one of the worst aspects of it and once they finally took a breather from heavy romance subplots in the last two seasons, the show improved.
There are good examples of romances between characters like Robin and Starfire from Teen Titans, even though they did pull the usual not ending up together until the end of the series card there. While I think Young Justice is completely overrated, I do remember liking the chemistry between Artemis and Wally once they became a couple. A good romance should involve characters with good chemistry together, so that making them into a couple doesn't feel forced, and it shouldn't be the main focus point of either character. It can be a good way to flesh out their individual personality traits and help them to grow, but they should ideally still get focus outside of their relationship too.
One other pet peeve of mine with romance is how they often have one character bully another and then at some point claim that they're a bully because of their feelings. This usually seems to happen with younger characters like with Helga from Hey Arnold, but I still really dislike that approach. It just reinforces the idea that boys picking on girls, or vice versa, are doing it because they like them and that's just a terrible message to send for multiple reasons. Avoiding that would be a good thing to do as well.