The black frames unify the different artwork which I think is helpful since there isn't really a color scheme going on. The breakfast nook or kitchen is often a fairly casual space in a home. It seems like a logical place to display kids' artwork. It's displayed and obviously valued by the family but doesn't overtake the decor if you prefer to have a formal living room free of kiddie stuff.
Most of these frames were purchased at IKEA or Target fairly inexpensively. I've also posted previously about thrift store frames. They are cheap, abundant and easily freshened up with spray paint.
This piece above is hung in a hallway.
This robot graces a kid's room corner. It's hung a little lower than normal so little people don't have to crane their necks to enjoy it.
Although I'm not very good at tossing artwork, framing a couple of pieces each year makes it easier to let some of the drawings and paintings make their way to the recycle center.