I'll first say that I know there are differences when in different, otherwise NORMAL, shoes. I never advocate getting a special pair of shoes that have abnormally high heel or insole, I never advocate putting things in the shoe to make a child taller. Height limits are for safety, so advocating *special* things, that would otherwise NOT be worn, is NOT something I can abide by.
But what YOU are talking about is a simple choice between two NORMAL shoes that would normally be worn. And you just don't want these normal shoes, which he would wear all around the parks anyway, to be soaked.
If you were asking about special shoes, I'd yell "safety" at you, but you are not talking about special shoes, just choosing between two normal shoes and not wanting his feet to be sopping.
There is a measuring stick at the beginning of the line, and one at the end, after going down the ramp just as you're about on the boarding platform. The spot where you measure again is still, but once you're on the normal platform, it moves. It's hard enough to walk while on a platform going around let alone change a shoe.
Therefore, the only time you'd have to change the shoe would be ON the ride, when everyone is strapped in. The rafts move and if you hold things up by changing shoes before strapping in, it will cause an ultimate ride-stop...not very nice. And it could cause the CMs to wonder what is going on, which is unwelcome attention.
So then you'd be changing shoes while on the ride. I've seen people take OFF shoes, but if a CM sees the bare feet they'll yelp at you about it.
I would probably stop by GRR early on, when it's NOT planned to ride, to see how he measures up in his sandals. If he's fine in the sandals, he's fine!
If he is fine in his tennis shoes that he'll wear everywhere, you have a choice. Either change while on the ride, or...since this is the ONLY 42" ride, just let him wear his sandals everywhere else, and let his tennis shoes get soaked and leave them on him for the ride!
Actually, that last one is probably the BEST solution. DS loves wearing his
Crocs...since Crocs fit wide (on normal width feet), Ds can easily get his wool blend socks on for mornings and evenings, taking the socks off when it warms up. So it wouldn't be strange for your guy to wear sandals around the parks, and so if his shoes get soaked while on GRR, it's no different for him to change to his sandals as it is when I change to my sandals after a watery ride.