the 7 is a smaller, lighter action. it differs from the 700 mainly in the rear bridge area. they require different bases, and different stocks.
typically, 7's come w/ shorter, lighter contour barrels than the 700.
other than that, the two are essentially the same.
for a youth gun, it is awfully tough to beat an m7, though if you get the 700 and a wood stock, you could cut the stock down in 1/2" or so increments, save the pieces, and as the kid gets bigger, glue them back together to get the proper fit, until s/he's done growing, at which time you'll probably want to buy a replacement stock.
it is usually harder to get an m7 to shoot as well as a 700, but most often it is because of the lighter barrel (varying the pressure point on the barrel might well cure that). the m7 is certainly big game accurate to a few hundred yards, and it might drive tacks, though the 700 will generally outshoot it. no wrong choices between the 7 and 700.
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