The LH rule is supplanted by US law, which requires a mask on all passengers age 2+. Various exemptions and means to obtain those, but the bottom line is that the US law applies to all arrivals into and departures from the US, having nothing to do with the "flag" on the aircraft.
The broader issue is that there are a lot of good materials out there on how to make masks a "fun" part of the trip and a challenge for little kids. Ultimately, you can't do better than your best.
https://www.cdc.gov/quarantine/pdf/M...01-29-21-p.pdf
No idea what the rules will be this summer and whether enforcement will be lax or strict.