B.A.
Mirafiex frames are designed to be as safe as possible for kids. Flexible plastic, no metal parts, etc. Whether or not they are necessary depends on you child. Some 4 year olds take really good care of their stuff and aren't extremely active. Some never stop moving, and mirafiex frames are great for them.
Pay special attention to the lenses, Lenses often cost more than the frames, so you'll want to get a scratch resistant coating on them.
I wouldn't let comments about trusting us more than the optometrist bother you. I'd wouldn't ignore a lens recommendation from an optometrist, But they have an incentive to sell more expensive frames. Don't be afraid to shop around. A lot of different places sell Miraflex frames. Shop around and find the best deal