Man, it's a great idea and the kids will love it, I wish I could have been at such an event in my time watching PewDiePie in the open air with a can of soda and a packet of crisps. Try sites like this https://easytube.pro/ and download in the best quality possible because the image will be stretched on a large plane, ideally double-check what it will be exactly on the projector and most importantly be careful with the weather because nowadays you can't trust weather forecasters ha-ha.