I love my Mariposa. I have never tried a Prophet but I think the answer to your question is that the stiffness of a closed-cell pad provides plenty of stiffness for a pack. My Mariposa without the carbon fiber stays is still plenty stiff to transfer weight well.

But let me make an observation about kid's packs. I've been thinking about this recently because I have 5 kids and so I'm also in the market for kid's packs.

Take a look at the picture of the girl wearing the Kid's Prophet. Now take a look at the hip belt. It's just an unpadded strap, designed to keep the pack from bouncing around. It's not designed to take any weight (at least not comfortably). I don't think this is a design flaw. I think the MLD people have thought long and hard about this and come to the conclusion that I have and that is that a kid's pack shouldn't really transfer weight to the hip like an adult pack. So a nice padded belt like a Mariposa has isn't all that necessary.

Now I'm no expert on pack design but when I look at my kids that are of the age you are talking about this is the conclusion I come to. Their hips are so narrow that I can't see a hip belt working. For my 14 year old it would work, but not my 9 year old.

So I think I'd recommend a different strategy. Rather than the Mariposa (which is one of the bigger packs in the Gossamer Gear lineup) take a look at the Whisper. It's a smaller pack. It doesn't look as rugged as the Kid's Prophet but without the optional hip belt it's half the price. Even with the optional hip belt it's 3/4 the price of the Kid's Prophet. That's probably the way I'm going to go.