I'm hopelessly in love with fixed power scopes but I will never say they're "better". Honestly, a variable is far more versatile... but I love a fixed 4x still.
The thing with kids, and my four kids are still a bit young, they have a bit of a "peripheral vision learning curve". You didn't mention your child's age but assuming this rifle is, indeed, for a child?
The younger the child the more likely they are to have a bit of tunnel vision. Their little brain concentrates on one thing and the rest of the world sort of blanks out. Anyone who has ever had to yell at a child knows how they can totally tune out your voice and flagrant gestures. As a child grows their brain starts to open up a bit and they notice more subtle things but this really depends on the kid, not an age range.
If you put a younger child behind a scope they're only going to see whats in the crosshairs, not the person or danger, safety wise, that stands just outside of their scopes field of view. ...heck they may not see it even if it is in their scopes field of view but not directly in the crosshairs. Iron sights really helps with this issue because their field of view is so much wider, they can see a person walking up form the side.
The problem, in my opinion, is that; iron sights require a good bit more practice to be proficient- especially with a child, or someone of any age for that matter. Also, many youth type rifles are not offered with iron sights and very low power scopes are less common in our high-magnification age.
Having said all that...
If the rifle offers iron sights please let them shoot it as much as possible with open sights, they'll learn that there's possibly more in the bullets path than just their target.
If the rifle does not offer iron sights then use a very low powered scope and, if it's a variable, make sure they keep it on the lower settings.
Hope this helps...