DS 6 has been wearing one for months, no problems, no fuss.
The first time he wasn't keen on the type of mask, and walking uphill wasn't pleasant so he cried. We found him some different styles, and also made sure they had fun designs on. Since he got those, he wears them very happily, and with the different style, there's no problem even running around in them.
He was born with an airway defect meaning his airway can collapse, and also has asthma. Still no issue. Also means if he's exposed the viral load should be lower, meaning he has more chance of surviving, because if he catches Covid, he has a high chance of serious complications.
As a scientist working in both contaminated zones and clean rooms, I've frequently had to wear masks for long periods. Despite having asthma, I've had no issues wearing them, and I highly doubt we'd have been permitted to do/capable of doing the (highly potentially dangerous) job and calculations we were doing if anyone thought we weren't fully with it due to hypoxia.