Future / present proofing your pond sounds a very sensible idea. It may not be your child that falls in but imagine the horror if at a party a friends little brother fell in while their mother was watching someone else, even if all that happened was wet feet forgotten about once sweeties are mentioned.
Yes it's fine for us to say "oh its not really going to be a nature pond" or try and impose our suggestions on you but ultimately its for you to decide what you think is best. Its your garden that means you have ultimate control. The wildlife will just learn to adapt as no doubt is has done many many times before!
6" square mesh seems plenty big enough for wild life to me (esp. for frogs and things). After all if hedgehog was able to get in through the grid surely it can get out through the same hole? Its also a good size to get your hand in to retrieve action men who claim on the television that they can swim but actually sink like stones.
I'd suggest seeing if you could get marine grade 316 stainless steel (316L being a low carbon content, 316H being high carbon content) rather than galv or painted that should prevent issues with rust contaminating the water. Now you could google the life out of this and you'd find that it does react slightly in warm sea water (causing brown staining) and but this shouldn't apply to your situation. As with all materials there will always be a slight risk you have to take. This is the grade that as an engineering type bod I have used all the time for systems that are outside / get washed down regularly and its used for boat fittings which should go some way to proving its water friendly properties.
One thing to consider is how it is made removable so that you can do maintenance on your pond (taking out leaves, removing dead plants / fish).
Hope that helps.