Hello Fillyjonk, my wife has asked me to write down how we explained questions about the sun to our daughter
This is the explanation we gave to Indy:
We think the big bang happened because if we look through telescopes that all the galaxies we can see are moving away from each other. As we know that they don't just stop or change direction, it should be valid to trace their path back to find out where they were in the past. if we do this it turns out that all the matter in the universe was at the same place at some point in the distant past.
The obvious questions about this theory "what happened before the big bang?" and "what did the universe expand into?" are really founded on our intuitions about how things work based on the world around us.
General relativity shows us that time and space are intimately bound up with matter, and aren't the static backdrop we expect them to be from everyday experience.
So really those questions don't make sense, time and space (as we know it at least) were created alongside all the matter in the big bang.
Stars are formed from clouds of hydrogen and helium gas out in space under the influence of gravity. Gravity pulls the gas together, until it gets so dense and hot that nuclear fusion can take place.
I think I described the sun as "a big ball of gas on fire" which I think is reasonably accurate (the "fire" obviously isn't fire as we know it which needs oxygen etc, but that's a distinction I'm leaving till later), and something a 4 year old can easily visualise.
Stars originally formed as the early universe cooled and became nonuniform (lumpy) - the denser parts gradually became more dense, and formed the stellar nurseries where stars were created.
The picture I have in my head here is imagine you are holding a sheet taut and horizontally, now imagine you somehow place lots of marbles closely but equally spaced in a grid on the sheet. If the spacing was exact, then nothing would happen, but any small inaccuracies would mean that two marbles would roll together, which would then pull the sheet down, and surrounding marbles would then be drawn in, and what you would end up with is dense clumps of marbles being formed.
And stars are still being formed in the same way in nebulae today.
One useful link to everyday life is that any elements other than hydrogen and helium were originally formed in the heart of a star, and anything more dense than iron was formed in supernova. I explained to her that the gold in my wedding ring would've originally been formed in a supernova.
hth
There is a useful, but old, book we have called Frozen Star by George Greenstein, which explains about stars, black holes and pulsars.