ESSAY
Why Damian, 10, can have something resembling an adult relationship (in feelings) and Dairine, 11, cannot.
Damian was rarely (if ever) shown affection, and no one in his entire life has had a stable, committed, monogamous relationship. He doesn't know how they work, or how they're supposed to work. He doesn't know about love, except in literature. It's all completely walled off from him.
Damian has also never had a childhood. I think that much is evident. He has lived his whole life under the expectation of being a small adult at all times. Kids with childhoods don't kill people, and they don't learn how to field strip an AK-47 when they're seven. Everyone (who raised him) expects him to be mature and adult and so he expects himself to be, too. But he doesn't really know how. He doesn't exactly know what it means, except it means people don't make fun of you, and they don't laugh at you, and they take you seriously when you speak. And hypothetically they're in relationships. Not that he, personally, would know.
There's been some speculation in fandom that Damian was artificially aged. I'm not sure I agree with that but who knows. I'm pretty sure he's growing at a normal rate. However, I would not be at all surprised if his hormones kicked in early. He's in peak physical condition and genetically advanced. I'm not totally sure how those things work BUT IT SEEMS REASONABLE? He's made comments in canon about girls (asking if Katana was seeing anyone; talking about how ~magnificent~ Cass is) so I don't think it's entirely outside of his thoughts. I think he's probably thinking about things a lot more, but also super worried about whether it's ~normal~ or ~what he should be thinking~. I mean, he probably thinks about killing people a lot, too, and it's been made clear that that's unacceptable. Why should liking someone be different? No one else talks about it or displays it. He just doesn't know.
So, really, Damian is not a typical child, at all, and I think it would stand to reason that his development in this arena would not be typical. Without suggesting that I've spent much time at all pondering it, I do want to point out that I don't think he's at the more… physical stage. I think that's still a few years off, for sure. But I do think he's at the psychological stage. It's very clear that his mind and his body have been developing at a different pace, and it's screwing him up in a lot of ways. He doesn't know what's so great about kissing, and he doesn't know why adults do it all the time. He doesn't want to do anything remotely sexual with Vanya (THANK GOD, AMIRITE) but he sees that adults kiss and so he thinks it's something he should do to show how he feels. Or shut her up. Either/or, really. That's why it's restricted to on-the-cheek or, if he has to, a peck on the lips. Everything he does in this relationship is because he's a) mimicking what he thinks is the correct course of action, without having any knowledge of it; b) covering for his embarrassment and projected humiliation; or c) doing what he actually wants to do. C is the rarest.
Which honestly brings up the embarrassment factor. Damian actually doesn't know if being in a relationship makes someone weaker. He thinks liking someone gives someone else a good chance to target them, but he honestly just doesn't know. He does know that he has no idea what he's doing, and that makes him weak. Adults always know what they're doing, right? And, as previously stated, he thinks he needs to be an adult, so floundering undermines that. That's why he doesn't want Tim to know. Any slip-up of perfect control and exercised command of his life is an open invitation to start challenging other things that Damian controls or needs to control. It would be the perfect opportunity for someone to see that he doesn't know what he's doing in any other aspect of his life. Because he doesn't. He is very, very lost, and this thing with Vanya just puts the spotlight right on that fact.
To address the title, Dairine has none of these hang-ups and is generally well-adjusted and pre-pubescent. Also has a stated canon aversion to liking people that way. Damian is a very, very special child.