Charlie was obviously here! He was sporting this outfit today and while he didn't stay at the grill the entire time, he was definitely there for your grilling needs early on. At some point, he'd have engaged in a water balloon battle with the kids. He may have not so stealthily purposefully thrown one at Bill's back when none of the kids were watching (because he'd have told them to only throw them at people playing) and smirked at Bill.
Albus would have arrived with Credence, Luna and Helena but a little later into the party; since they had to go to Luna's graduation first. However, he definitely showed and as soon as they were on site, Helena basically ran away from her family to join the fun. She had her eyes on taking you down in a water balloon fight, Conner, apparently.