You could do it in front of the castle, but you're going to have to stand pretty far back to get them all and there will be a lot of people in the picture. Can you have them stand 8 and 8? You might not get a lot of time right in front, but you could try moving forward, like on the curb right in front.
I'd try:
Castle, do this about halfway through Magic Morning when people aren't passing through a much.
Esplanade, after the first hour there's not a lot of hopping happening yet, you could get either park's gate in the background.
Main St train station steps, also not a high traffic area once the park opens, so head their first.
Sword in the Stone, there usually aren't more than 2-3 groups waiting for a picture there. Queue everyone, send them in single file, and wait for those few seconds when everyone else clears out.
Mickey's Fun Wheel, during the day, the WOC viewing area is pretty deserted.
Storyteller's statue, can be hit or miss. Sometimes there's a CM there with a tripod, but even then there's not always many people waiting. But again, you'd have to stand pretty far back to get 16 in a single line.
Grizzly Peak, there's a little alcove near the waterfall on the Bugs Land/Cars Land side. There's usually a CM there for photo ops, but the view of Grizzly Peak never really caught on as the icon of the park that Disney thought it would be.
Greetings from Cars Land sign, you'll have to wait a little, but there's also rarely more than 2-3 groups waiting here. I think 16 kids would fit the width of the rail there.
