Kids love this shit as long as you keep it at the ELI5 level and stop when they are done and lose interest. My kid will throw around words like "microorganism" and "bioaccumulation" because I actually explain biology concepts when he asks. The other day he had a question about atmospheric composition and he was absorbed for about 5-10 minutes, complete with looking at molecular diagrams, and then he was done and went off to make his Lego people fight each other with flamethrowers.
If you have knowledge, share it with kids and let them see you enjoying science. They absorb more than you might think.
What you are describing is not osmosis, it is capillary action. Capillary action is caused by the forces between the water molecules and the molecules of the tube overcoming the force of gravity. You can read more here: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Liquids/Capillary_Action
Briefly, the water molecules are attracted to the molecules of the tube by adhesive force. The liquid molecules are also attracted to each other by cohesive force. The interplay of these forces causes capillary action.
However, it seems that tree sap moves by more than just capillary action. If you scroll down part way in this book they talk about it a bit: https://pressbooks.online.ucf.edu/phy2053bc/chapter/cohesion-and-adhesion-in-liquids-surface-tension-and-capillary-action/