To the above lists I would that for children their ages, touching themselves is very normal, because they've worked out it feels nice. It can also be a comfort mechanism for children who are neglected and deprived of normal interaction (you might have heard about self-stimming behaviours like rocking backwards and forwards resulting from neglect, but masturbation serves the same purpose). However masturbating excessively or in public can throw up flags once they've reached a certain age (I don't expect a 3 year old to know that touching your willy in the supermarket isn't appropriate, but I would expect a NT 7 year old to know that)
'I'll show you mine...' is usually normal in a playful-on-both-sides manner. The same activity accompanied by coercion or threats to keep it a secret from one child is worrying. Use of force with another child should raise massive red flags.
Language usually confined to childish terms like willy, not adult/explicit ones. A 7/8 year old might talk a bit about sex, and they will have a small amount of knowledge eg. 'that's how babies are made' or 'it's when a man puts his willy in a woman's vagina', but their knowledge of the mechanics should otherwise be limited, and they shouldn't be using adult terms eg. calling it 'fucking'
When it comes to things like what the SW said re. the hairbrush. When it comes to objects, it's normal to have a good giggle at objects which look like willies, or perhaps even with a hairbrush, holding it in front of them and pretending to pee. So for instance, laughing at a sausage because it looks like a willy is age appropriate. However, the same child then picking it up and simulating oral sex is a big red flag.
Persistant talk about sexual things is relatively uncommon, as is persistantly using sexual themes in play. Most children are curious, but not obsessive and can be distracted or redirected relatively easily.
There is a broad spectrum of normal, but be wary of adult knowledge of the mechanics of sex (beyond willy+vagina=baby), adult language, anything which they couldn't have learned from similar age peers, and anything which gives you a bad feeling - some things can give a very strong 'this is not right' gut feeling which you should listen to if you get it IMHO. A child's tone of voice, body language and facial expressions can sometimes raise flags when a child does something which you might otherwise have put down as within the spectrum of normal.
I am more than happy to advise further, you can PM me if you want. You can also find guides online, including several 'traffic light' guides to sexual behaviour in children. Google something like 'sexually appropriate behaviour children' and you'll get a lot of guides, a lot of organisations publish them.