Looking at the far bottom right pic in that ^^^^^^ Ortlieb conversion doc, you could install it as a pannier, and carry a small child on the outside on the backpack straps. I wouldn't let Child Protective Services see it, however...

BiKase brand laptop convertible backpack pannier:


The straps tuck in behind that panel for pannier mode, and the hook tucks behind for backpack mode. You can clip one strap to the other side's ring, and tuck in the other strap, and effectively have a sling bag. There's no stiff panel, so it flops some when used as a pannier. That long vertical zipper is for the is for the laptop compartment. Holds a 15"-er easily, and a large-is lunch cooler and 44-oz water bottle in the main compartment. Coffee travel carafe in the side compartment.

My original Targus beater:


Guts from an old CyclePro pannier, including a stiff panel that all the hardware is bolted/riveted to. This one would probably hold a 17"-er, and other business goodies of course, but no space for a lunch. Shoulder strap on the d-rings. Yes, that's a section of innertube for the bottom hook; I like those for the combination of strength and elasticity.

I also have a Timbuk2 convertible laptop backpack pannier. Pannier hardware is on the side opposite the backpack straps (similar to the Ortlieb conversion above). Overflap flips one way to cover the backpack straps when in pannier mode, and flips the other way to cover the pannier hardwaree when in backpack mode. Good solid retention using a fairly stiff panel, and a big main compartment with a laptop sleeve built in. Nice and big and solid, but I can't recommend it. Overflap is also a flat compartment, but otherwise it's just the one big compartment, no pockets for little bitty things, no little pen straps built in, etc, like you see in business bags. And a bit unwieldy as a backpack; it looks like a black version of the big packs the moon astronauts wore.