Another welcome. Bills ? about muck should be investigated. If its got lots of muck and you put cute little KOI in the small upper pond, they will end up in the pond. Once in the pond they will root around and get lots of sediment suspended in the water column. Might want to put in some hardy forage instead. I hate Koi, just fancy carp !!!
If ya got 10"s of ice and your area usually gets heavy snows and or snow that sits for awhile, you should consider aeration.
Dman has a good eye. If you're wanting to swim, that looks like a fine spot for a beach. If the slope is too steep ya might consider a pier with a climb out latter.
Might want to look up a pic of Duckweed. Duckweed floats on top like that but will appear quickly and the same with it disappearing.
For a 5 yr old, that's a long att span.
Good luck and stick with it, ponds and kids go together.

Do nature a favor, spay/neuter your pets and any weird friends or relatives.