Children exert their independence, or try to, at various times all through their childhood.
This is normal behaviour. How else could they possibly learn about social boundaries, right and wrong, what foods they like or dislike?
The ways in which they do this are not always easy for the grown-ups reponsible for bringing them up or teaching them.
With small children I have always found that making as little fuss as possible usually solves the problem quickly.
Remember the child who had heard a rude word? The first time we heard her say it, we said, "That's not a nice word, please don't use it." After that we ignored it as much as possible, didn't we?
It is no-one's fault that a child refuses to eat certain things.
I would worry if a child only would eat one thing for more than a day or so, but it seems that many of those who do so, come to no harm.
The child who ate her toast, after refusing it, when sent out to give it to the birds in the garden is a marvellous illustration of this sort of thing being a bid for independence.
Mummy wisely did not insist that the toast was eaten, so the child went out, offered the birds it, and when they didn't want it, ate it herself. Probably coninced her mummy couldn't see, hear or guess what she was doing.