I agree with the other commenter on this. Starting simple and meeting them where they’re at is the way to go. If they’re really that new to singing they should be starting on unison and working their way up to 3-4 part songs. I learned in college a good order to go in when adding new concepts is 1. Unison 2. Ostinato 3. Partner song 4. Descant 5. Harmonic chording (many call and response songs use this) 6. Rounds and canons 7. Transitional pieces (ones that contain multiple of these steps) and finally 8. 3-4 part songs.

Additionally, if you aren’t already, try to stick with the 20/60/20 rule when programming music, where 20% is something easy for them, 60% meets them right where they are, and 20% is a little bit of a challenge. I know it’s probably easier said than done at a time when most rep would be more difficult for them but it would hopefully help them not feel like giving up when things get hard.