@lightand
pointers such as
do they choose one language over another?
do they speak later, and with fewer words initially?
I presume young children mix the languages together frequently?
as you can see, I am clueless
The language they are most exposed to is usually favored. Children will often associate a particular language with particular people so if Mum speaks English and Dad speaks Arabic you might find your grandchild tells Dad off if he speaks English.
There is no reason to speak later, most children in the world grow up bi or multilingual. Children brought up with a sign language usually sign from a younger age and have a wider vocabulary but this is because it is easier to make hand shapes than clear sounds.
Very rarely will they mix up the languages, there are a couple of times they will eg if they don't know the word in the language eg when I was about 10 we had neighbours with a Dutch mum and English dad, the only time I heard their little boy mix up words was when he was talking to my mum about the vacuum cleaner. His mum did the vacuuming while dad was at work.
There is also something called 'code switching' this is used between people who are bilingual or people who share a dialect.
So lets say you make shortbread for your grandchildren, it might become known as 'Nanny's shortbread' and it will stay that way when your grandchild is speaking their other language.
Bilingual books can be found in the UK and you may also get them from the other language's country (ies).
He is very young but once he starts reading he might like to have the books in both languages,
Reading / writing can be a bit tougher particularly if the language has a different alphabet. It's easier if the other language uses the same alphabet and is phonetic, in which case you teach the child English reading and writing and they will pick up the other.
If you want to do some reading your local library will probably have some books, David Crystal's 'The Cambridge encyclopedia of language' is a good place to start if you are interested in language generally,