Theory:

     ‘Yes, I was so excited at the time, packing and saying goodbye to people and thinking about the ship I was going to sail on that I simply forgot all about it.’

     ‘Yes’, he said. ‘It’s twenty-five years, but it may still be there. Unless someone else found it....’

     ‘Would you like to go and look?’

     ‘I can’t climb trees any more.’

     ‘If you can’t, I will go and see. You just sit here and wait for me.’
Explanation:
  
He answered that it might be still there as when he had to leave the place, the man had been very excited to sail on the ship, packing for the journey and saying goodbye to everyone, the man had merely forgotten whatever he has stored in the hollow of the jackfruit tree.
 
It has been twenty-five years, and it should be still there unless and until someone else has taken it away.
 
She asked if he would like to go and have a look at the hollow of the tree.
 
And then he said he can't climb trees anymore.
 
The girl got so excited that she said if he can't climb the tree, he can sit there and wait, and she will climb up to see.
Reference:
State Council of Educational Research and Training (2018). Term-1 English Standard-9. I Can’t Climb Trees Anymore - Ruskin Bond (Slightly adapted) (pp. 157-160). Published by the Tamil Nadu Textbook and Educational Services Corporation.