Can my child change on the pool deck or use a phone in the locker room?

No to both. Under the MAAPP:

• Deck changing is prohibited. Anyone changing into or out of a swimsuit, when wearing only one suit, must use the designated locker room or changing area. Clubs must provide a private or semi-private space for minor athletes who need to change.
• No recording in locker rooms. Using any device's recording function (photos, video or voice), including a phone's, in a locker room or changing area is prohibited for athletes and adults.
• Adults in the program must not shower with minor athletes, except for a rinse while wearing a swimsuit or under the close-in-age exception. You can ask in writing that your child not change or shower alongside adults, and the club must follow that request.

Recommended: if you go into a locker room, use one for your own competition category and tell a coach first.