I planted them out early, long before last frost. They didn't even mind a deep freeze. It's the heat that kills them, so best to plant before the weather is too hot. I planted mine when tiny, too, maybe 2 true leaves. I just planted in hunks.
That was in 2006. Never planted them after that because they were BIG plants, died when weather got hot, and I had to yank them leaving big holes in the garden beds.
I tried a few California poppies last year, they took the heat very well (though it wasn't a hot summer). I have some volunteers that have been out there since fall and they didn't mind freezing winter weather at all.
Just plant out soon as you can reasonably handle them.
Karen