Related: Why did people appear to remember what was going on at the time they left England when they returned from Narnia?
The books aren't very explicit on this point, so here are a few possibilities.
Their time in the "other world" takes on a "dreamlike quality"
While they're in one world, the world that they just left has a "dreamlike quality" - there was some sense in which Narnia was like an extended dream while they were in England (and vice versa for England while they were in Narnia).
This would explain why, for example, they seem to have remembered details about what was going on when they left England when they returned. They evidently didn't remember the lamp-post until after they got back to England (prior to getting back to England, they only had a vague recollection of having seen it before), and they didn't seem all that disoriented when they got back to England. They appear to have remembered where they were and why they were there. (They even remembered to apologize to the Professor for losing some of his coats). This is much like you remember your whereabouts when you wake up from a dream.
The influence of the world itself
The world itself seems to have a good deal of influence on the characters' behavior, thought patterns, and even skills. It will often take hours or days in Narnia for characters to regain all of the skills that they had gained last time they were there.
This suggests the possibility that, when they return to England, they don't really retain all of their adult-like traits or skills, which would seem to make it easier for them to transition back.
They did have trouble, but the books didn't address that
The books actually don't describe much about their lives outside Narnia. In fact, in most cases, the books spend only as much time in England as is required to set up the story. (An obvious counterexample, though, is when Caspian and Aslan go to England briefly, but even then they're clearly "out of context" - that's fundamentally Narnians setting something about our world right, not just a story about England). After all, the books are the Chronicles of Narnia, not the chronicles of the characters' lives in England - the books are fundamentally about Narnia. One could argue that, in one sense, they're even more about Narnia than they are about the characters themselves - i.e. the books are fundamentally "about" things that certain people did in Narnia, not merely about those particular characters.
Genuine plot hole
It's quite possible that this is a genuine plot hole, and that C. S. Lewis either didn't think of it or didn't think that it was important enough to the stories to address. (The latter point could be related to the previous point - i.e. it's not important because the books aren't about the characters' actions outside of Narnia).