To my understanding there are two issues here:
1) The phone. Which you seem to be fine with, in the specific situation.
Where I live it is fairly common for 7-8 yr olds to get their first phone. (Too early imo.)

2) OW "harassing S7". Have you spoken to S7 about it? How does he feel about her texts?
If he feels as you describe I would definitely talk to H about it if I was you. Perhaps talk to OW about it.

Me: 34
Stbxw: 30
D:5 D:3
Mini bd: May/June 2019
Married: Aug 2019
BD: 6th Dec 2019
OM Confirmed: Feb 2020
March 2020: I filed for D
Waiting for D to be finalized and W to move out end of January 2021