Thanks everyone for the encouragement!
I went to see said horse, who was all she was advertised to be: 19, gentle, experienced, been there/done that. Round pen, arena, and trail ride test drives were all very satisfactory. In fact… fun!
Having the vet out to see if she has any major problems pending, but she appears to be very solid and sound, barefoot all her life (like that!) and gave a good buck and fart in the round pen- not worried about herself.
Will post back if this works out… hard to admit we’re not as young as we used to be, and maybe don’t need the young horses we rode ‘back in the day.’
Child will have regular lessons… but this horse seems to be a bit of a nana type, of a nature to let him learn on her in the barn and having a little fun together too.