Mine get to spend every summer outdoors and start basking
within a day.
I wouldn't worry about the mosquitos. I wouldn't cover
the pond other than a small area of shade. I have a 165
gallon preformed pond and cover about 25% of it with a piece
of wood for privacy and additional shade.
The 2 main things you need to consider if they will
be outside without your supervision are...
1. You need to enclose it with something to keep predators
out like raccoons, dogs, cats and yes even kids. I have
my pond enclosed completely in a 6X6X6 chainlink dog
run. Also, you would need to provide some kind of
drainage for when it rains. I have holes drilled at
at certain depth so the water level never rises above
this level.
2. You need to make sure that it does not get sun exposure
all day so you do not overheat your turtles. Mine is
set up so it gets sun in the morning and late afternoon
otherwise it is in the shade. The enclosure is on the
East side of my home.
My way of doing things may seem a little over the top but
I can put my turtles out and never worry about them during
the summer which is very much worth a little planning.
Linda