What's the fastest way to populate a shared album?

I want to create an album containing all the best photos taken by our family on a trip. What's the easiest way to collect all the photos in one place?

I created a Shared Album and Kid accepted the invitation. If Kid selects a small number of photos (e.g. 10), he can add them to the Shared Album immediately and they replicate as expected. But if Kid tries to add a larger subset (e.g. 100) at once, his phone shows a "Preparing..." dialog that takes forever and then bombs out.

Is this expected behavior? Is there a better way than adding them 10 photos at a time?

Alternatively, is it possible to do this using the web iCloud interface since the photos are in the cloud already anyway? I logged in to Kid's iCloud account from my mac but iCloud Photos on the mac doesn't seem to recognize Shared Albums at all. Am I missing something?

Me: MacBook Air running Photos 3.0 on High Sierra (10.13.6). iCloud Photo Sharing and iCloud Photo Library are on. Download Originals to this Mac is also on with all the photos on an external drive.

Kid: iPhone 5, running iOS 10.3.4 (the latest for this device). iCloud Photo Library is On with the Optimize Phone Storage option selected. iCloud Photo Sharing is also on.