I've had good luck with Eterna brushes (I think they might be a student line of Grumbachers, not sure). The #8 flats are very durable ana aflexible size for tempera. Also Delta #340 bristle brushes are good. For finer work with water based paints I like the Royal RG 590 rounds in a size 6 or 8. They hold a good point for several years of use if they're taken care of.
I spend an entire lesson on brush construction, care and technique (which I would be happy to share) before I paint with ANY grade level. I always save ruined brushes to use in my demos. Kids need to know WHY the brushes can't be stored on their bristles and why they need to be used gently (not scrubbing). No one wants to use the stiff and splayed brushes. I walk around the room and point out positive behaviors when they first start painting and as they clean up. By the end of that first lesson most of them get it with a few reminders. I enjoy seeing them police each other at the sink when they wash up.
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This lesson would be of interest to me and to my students, please share. Thanks so much, Marianna