Elementary-schoolers write their lowercase a's as single-storey while typography nearly universally uses double-story. They write their lowercase g's as single-storey while fonts often use double-story. And they write their uppercase Q's using a tail that crosses the bowl, rather than one carefully underneath, as if often used in typography.

So I wouldn't rely on how elementary-schoolers write as a guide for typographic design.

In any case, there is a perfectly good reason to omit the crossbars, which is consistency. They are essentially serifs, and it's totally inconsistent to use serifs for just one out of 52 letterforms and not on the remaining 51.

Putting crossbars on an I is like a bar through a 7, a slash through a zero, or a bar through a Z. It's a hack for enhanced legibility but it's ugly. It's noisy. It's distracting. We do it in handwriting or in monospaced code fonts when required, but not for professional-looking body text. Sometimes the more aesthetically pleasing choice is the best one, which is something "proper design" takes into account.