I don't have kids; but I need to raise the 4200's bucket up high so I can drive the L130 in and out. When the bucket is up high, I engage the loader lever lock that Deere provides. It won't prevent a slow leak-down, or a rapid drop due to pressure loss; but it will prevent the loader lever from being moved. Once the L130 is in or out, I drop the bucket and depressurize (de-energize) the system.
As long as you have a safety brace should not be a problem , can't tell from your pictures . Lift the loader set the brace and set the loader on it . And those rods are not going to rust . We park machinery exposed and have had no problems . Those rods have a microscopic film of oil on them ,I have yet to see any rust .
For a safety stop all you need is a couple of pieces of angle iron cut to the correct length and ty-raps to hold them in place. We put two angle iron edge to edge on each piston, hard to explain, cross section would look like this: <O> You could probably use velcro instead of ty-raps so you don't have the hassle of cutting them each time.
You drive under a raised bucket? No way in ****...
You drive under a raised bucket? No way in ****... The only way I work on hydraulics over my head is with some sort of mechanical safety device. Jack stand, prop rods etc.
I dump pressure in my loader as well with it on the ground when it gets parked or left any significant amount of time.