Short and simple is always best. Thank you for your generous donation(s). With your help, PTA/PTO was able to earn $---- which will be used to support various programs (or insert the name of the program) at (name of school). Parent involvement is an important part of our childrens school experience. We appreciate your contribution and look forward to working with you in the future.
You can also put something at the end of your note letting parents know about your next meeting/event and invite their participation.
Just my $.02