We had an umpire (BU) that was constantly checking her phone throughout the game, which left her out of position multiple times. Should I have approached her during the game?
No, a phone is NEVER acceptable for a timer. You guys have covered that. (OK, maybe the umpire who let you know they are on the verge of a family emergency ... even then I would put the phone in the dugout and not keep it on me.)
While wearing a watch is typically considered a no-no for professional umpires, I would argue it is a necessity for youth umpires working tournaments. The argument against wearing a watch is that checking it, like checking a phone, looks bad.
Here is something that I recently picked up while working baseball tournaments where they do not provide us with a timer. (Yes, I have my own, but I also have too many experiences with the kitchen timers failing.)
Flip your watch face around to the inside of your left wrist. Anytime you look at your indicator you can quickly check the time also. People do not see the universal sign of "impatience" (holding up your arm and looking at the back of your wrist) -- they just see you checking the count.