I think drunk/uncoordinated adults are more of a risk.

I had a similar issue and I ended up rigging up a ladder that boards from the side of the boat.

I'll bet it's free if you're over 65 and on Medicare. Just ask Wilfred Grimley.

Mom looks good in the photo. Cobalt should hire Vanity Fair to airbrush her mommy pouch.

Depending on how you want it to look, this really is nothing more that an extra wide, single step, retractable ladder.

I beleive it has a safety switch so the boat can not be started when it is lowered.

The extending portion is where the cost will skyrocket. A fixed mount tubular style platform with integrated boarding ladder(s) would be the best option I think. Once parts start becoming in motion, and then need to extend and lock in multiple positions, the complexity and expense will go hand in hand. An option might be a platform with the boarding ladders facing directly pt/stbd underneath the platform. This would force people to board on the sides and never get near the props. Ladders like this for example:
http://www.armstrongnautical.com/pdf...dder_flyer.pdf

I seem to recall Nortech adding tubular swim platforms to a few of their cats.

Found a sample idea on a vee.

One could use that design and improve on it if they want it to lower. Be as simple as making the supports hinge underneath with a trim ram on each side to raise/lower the exrtention. Just have to make the platform keep level while moving, done by linkages
Care would be required to keep from hitting the drives, and of course a safety switch for a no start condition when lowered.
Piece of cake!

My take, cheapest easiest way go over to an rv store buy a step for a motor home reverse engineer it out of marine friendly materials and boom you're done. I'm sure you've seen em on the side doors of an rv the bottom step pulls out from under the rv and articulates down to an appropriate step level. I bet if you built a nice one you could sell them

thanks for all the ideas. if and when I have time to fabricate something i'll post pics.

My boys have been climbing up and down into my boat with no problem since they were old enough to want jump off the swim platform. I have extension boxes and no ladder. My props are also labbed, but not any sharper than stock props.