"Have you considered a good old-fashioned flip phone?"
We did. For $118 the unlocked 4a was the better choice. The current 4G VoiceLTE "dumb phones" phones mandated by AT&T are not as dumb as the past 3G phones like the old school Pantech 3G QWERTY phones.
Also, the two Nokia 4G phones we purchased (and promptly returned) refused to work on the AT&T network even though they were on AT&T's approved $G VoiceLTE list. Plus, the new Nokia software (KaiOS?) was horrendous and had advertising you could not shut off.
I guess the idea of the perfect kid phone is GrapheneOS in a small hardware package: large enough to be usable for meaningful communication; small enough to not beckon a child's attention constantly.