There’s nothing very explicit in terms of sex here. The first half of the book is focusing on a couple of kids after all. And the first romance we do get (the one between Maggie and Philip) is very innocent. They profess their love for one another (though Maggie is a bit reluctant) and Maggie does kiss Philip briefly.
You have to hang around until Volume III before you get any real romance. We’re talking about Maggie and Stephen, of course. Again, there’s nothing explicit at all. The two of them barely even kiss.
But this is Victorian England, so the fact that Maggie and Stephen are really sexually attracted to one another, and the fact that the narrative makes us aware of this, is pretty risqué. As it is, we have scenes where they gaze at each other, are acutely aware of each other physically, sit and stand to close to each other, and kind of weirdly bond over petting a dog (which is in Maggie’s lap at the time so, hey, scandal alert).