Kids ski binding remount? Move only one side?

I don't have a login for any ski forums so I figured I'd ask here.

My 4 year old has outgrown his 70cm ski's. My 6 year old has outgrown his 90's. 

The obvious plan would be to give the 90's to the little one, I just checked, they're the perfect length for him, and get the older kid (used) ski's that fit.

Trouble is, the 4 year old is tiny. His 14.5 ski boots actually seem to fit this year, last year he was swimming in them. Those itty bitty boots are too small for the 90's. I just checked and adjusted them as much as I could. They're set up for about 17.5 I'd need to move the bindings about 1.5cm or 2 closer together. I'd need them remounted, I'm at the end of adjustability.

So which would be best, move the rear binding forward or the front back? To minimize needing to remount both ends.

Forward is supposedly tighter handling and back is better for powder and stability? What's better for a learner who (fingers crossed) might venture off the single kids run this winter?

Will any of this even matter for someone who insists on skiing between my legs on the bunny slope and eating fries in the lodge? Counting helmet and winter coat, he might be 26lbs.

The usual practice is to move the heels forward. This keeps the toes and the balls of his feet close to the center of the skis for better overall balance. Moving the toes back would put him too far back on the skis, and little kids already have a strong tendency to sit back on their skis (this is normal, and they outgrow it as they grow older). There's also such a thing as being too far forward on the ski, so you still might end up moving the toepieces and the heelpieces. Take the skis to a ski shop, let them decide, and pay them to use their jig to remount the bindings properly.

If you move the bindings closer together, will there be enough room for the ski brake between the heelpiece and the toepiece? The bindings don't just have to make contact with the boots, the heelpiece has to push the boot forward into the toepiece with the right amount of pressure so the binding will retain the boot when it has to and release when it has to.

Are the bindings on the 90s designed for someone as light as your 4 year old?