I don't want to sound sexist, but I'm trying to picture the reverse. If a female student at the end of fourth grade received a private poem from their 24-year-old male teaching assistant, talking about how much he loves and cares for her .... the police would be involved. The same standards must apply regardless of gender or intent. If you get clearance from the classroom teacher and the head of the school (headmaster / principal), I'd advise writing the same generic blurb to all of the students. Then at the end of the generic blurb perhaps write one or two sentences of personal encouragement to each student.
Example: Blurb about how much you enjoyed working with the class, how you wish them all well, and hope they have a good summer. Then, "Annie - I'll always remember that joke you told me about ....", "Joey -- I'll miss helping you with your Math homework and hearing about (TV show)". Then for the student you are worried about "X -- I know next year will be a big change for you. Good luck and best wishes! You can do it!"
That's all you can do. The repercussions of inappropriate communication with youngsters can be severe, if misunderstood by the wrong person. There are even legal boundaries for licensed teachers who are in a union.
I would tread very lightly in your position.
Good luck.
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Beatles