I think if he really wanted to, he would’ve done it, but he probably doesn’t want to take the risk. The thing is, whenever it comes to scenes with those younger characters, it always feels like they’re treading carefully. That’s why, at the start of Slave, there was all that hidden dynamic with Jinora. But once that passed, and now that Jinora already has multiple scenes, there’s probably no more risk there.
I remember Marty mentioning he was doing something risky when he released the update with loli Korra taking the young potion; he probably got that same fear again. But it passed, and it was just one scene. So now, I think from Marty’s point of view, the current Ikki is already “enough.” It’s like she’s become the equivalent of the past Jinora, and he’s not going to want to push his luck anymore. That’s how I see it, at least.