Kids "Boat"

From the pictures, it looks like the upper and lower mortises are close enough to the edges that you can use a marking/mortising gauge and square. And I’m sure you know to lay out and cut the bottom ones before making the curves. But I guess you’re thinking about the steps that are midway across the width of the side pieces. You could try using a simple marking (not mortise) gauge to scribe your layout lines, as the mortise gauge my not have enough length to reach the center of that board. Make sure to start the lines in pencil and finish up with a gauge or knife.

If nothing else will work for you, maybe you could make a special tool for the purpose. A marking gauge is nothing more than a beam with a pin (or two) in it to scratch the wood and a fence to set the distance of the pin(s) from the edge. You could maybe put two nails in a piece of wood – the two nails set apart from each other at the same distance as the wodth of the chisel you will use to cut the mortise. Then attach another piece of wood to that one as a fence, placing it precisely at the proper distance from the pins to set them in the right location. Not sure if my words adequately relayed my thinking here.