Yes as the previous poster said, get a "recommendation letter" from a parent if you can.
Also, from other staff members/co-workers. That is the norm, even if you were working else where.
I would delete out the whole paragraph that says "I am very impressed with XXX Elementary and have thoroughly enjoyed..." Because, this is an "opinion" and subjective comment. Not having anything to do with your "job" or skills. And it can be taken as a "critique." etc.
Try not to comment about or use words like "relationships with my co-workers..." but use words INSTEAD like "I have had a wonderful opportunity working with the Staff of XXX Elementary, and have enjoyed being a "team member" with them, contributing to the overall outcomes and progress with each student and carrying out and supporting the school's goals." That sounds more professional and not just a "Mom" writing a paper on what they do.
Use key words like: contributed to the team, successfully managed....., was a part of the teams/Teacher's goals and success with classroom objectives, implemented...., supported success of the team's....., provided continuity for the students of their Teacher and carrying out....,
Show that you were a "positive contributor" to and for the Teacher you Sub for....
just some quick tips,
All the best,
Susan