Don't let them get hung up on the opening sentence. It's a tough one for all writers. Sometimes it's written last. Just work piece by piece until they have a first draft. Take another break.
Writing is strenuous mental work.
Kids need to get away from the first draft and do something physical. Later they can re-read it with fresh eyes and have some fun editing and polishing. Do a little assessment. Ask which sentence is their best. And don't forget to heap on the praise.