Are they the new leaves that are yellowing (possible nitrogen deficiency) or the lower, older leaves that are yellowing (possible potassium deficiency)?

Is the whole leaf yellow, or are the veins still green?

Sue

I'm curious as to why these aren't already in the garden in your zone? Well past safe plant out time. Or at least outside in containers.

Most likely it is far too hot for them to be inside and having to water them that frequently combined with the heat will stress any plant. Plant them in the garden and I'd bet they will recover. But for future reference, squash do best when direct seeded as transplanting often causes stunted growth. Just something to consider for next year.

Dave

PS: I'd think that it would already be far too hot in the greenhouse for anything to survive there unless you have a massive cooling system of some kind.

here are some pictures

yellow crookneck
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butternut
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You might consider that they're getting root bound. Carefully tip one out of the container and see if the roots are getting crammed in the bottom. Indeed they should be in the ground so that they're used to their growing conditions. Hope this helps.

They're short of nitrogen -- the indicators are the older leaves are yellow while the younger leaves are green.

As has already been said, they need to be planted into the garden. Right now, their roots are cramped for space.

For future reference, as long as outdoor temperatures are suitable, you can move seedlings to the ground as soon as you see the first true leaf beginning to develop.

I would agree and say lack of nitrogen and cramped roots..
I transplanted 4 kinds of squash from cups to garden and they were just a little smaller than yours and I can say that the root ball looked like spaghetti..mine all took and I had a head start on production. I can only imagine what your roots look like. Get them boogers in the ground now and hope they don't get shocked.

Happy Gardening