I am planning to take my 5yo to her friend's birthday party (pool party). Is there anything I should be mindful of? Can I let my child play in the water when there is a chance it might be dirty!

The pool might be chlorinated so I'm sure its clean. It may not be 100% clean but it kills the bacteria. Just let your lo know not to drink any pool water :-) My boy sometimes accidentally ingests pool water but he has never been sick. Just be mindful of him when he is the water and have fun! :-)