Where would you move to up north that doesn't have mosquitos in the summer?
Late Sept and October are actually lovely here, and I say this as a former 40 year California resident who absolutely loathes the summers in the DC area. It's the only time I actually like the weather here.
My friend moved to Chicago (from CA) in her 20s. Once she had kids, she moved back to CA because she couldn't handle not going outside in the winter for weeks.
When my kids were younger (they are teens now), I took them to the pool a lot. Just cover yourself in bug spray and take a shower at night to wash it off.
I will say, though, that now that my kids are teens and going off with their friends on their own, I rarely leave the house in the summer. It is depressing as heck. I do hate it.
Instead of winter depression, I have summer depression because I don't want to go outside. But, I'm priced out of CA for the time being.