I don't know if they are the lightest, but the Mizuno Watley 9 Spike is a very light shoe, and it wears extremely well.
With the front spikes being replaceable, your kid will probably out grow them before wearing them out. (Unless she is a pitcher lol)
The Mizunos seemed to be the lightest, but my little pitcher opted for the sturdiness of the Adidas...or maybe she just wanted to match her soccer cleats. lol