Pipbin what to you imagine they can do about it? 
I totally agree rooty the likelihood of her getting out any time soon is so slim it's better that the OP tries to integrate, accept her choices and make the best of it, at least for the time being. Playing the long game is the only choice she has.
Look, unless the OP is being beaten and abused by her husband and his family she doesn't have much of a case to ask for anything from anyone, certainly not the Saudi authorities and the British consulate won't be terribly interested either. They'll be too busy dealing with Brits imprisoned there on trumped up charges and the like.
mama and her H skipped the UK to leave behind a trail of of debt. She is married to a Saudi man and has run away to Saudi, where she (understandably and perhaps not unreasonably) is expected to live like any other woman married to a Saudi man, observing Saudi societal norms, because she's in Saudi. She chose to go, she wasn't kidnapped or forced. Her H was going to leave her and abandon the children and she chose to follow him and take her chances rather than face the music at home and now she's bleating that she doesn't like it, after a few months. I'm really sorry to be harsh but that the old cliche 'you made your bed, now you'll have to lie in it' springs to mind. 
Yes it's a very different way of life but nothing that she could not reasonably have expected given that she's been with this man for 8 years and presumably has read/heard a fair bit about what to expect when she got there. She doesn't speak much Arabic and has no friends yet so it is going to be hard for a while. The only way to make it better is to try to embrace it with every bit of positivity she can muster. There is no other choice at the moment.
mama I'm struggling to understand whether you want to be away from your husband or whether you just want to be away from SA and your in-laws, and the depressing lack of money and familiar home comforts.
I'm not quite sure whether you are avoiding admitting that he is controlling, abusive and oppressive towards you, (you have been rather vague and evasive on that so far, with lots of alluding to abuse but nothing specific) because you are embarrassed to admit that you went with him in spite of it, or whether you want to give the impression that he's abusive in the hope that it will provide you with an easier route out, and more help once you get home. The truth is that sadly no-one is going to be sending a rescue party either way. But if and when you do get home, you have young children so you will be entitled to housing regardless of your background and circumstances.
You said 'they' hit your son if he annoys them, but I'm not quite sure whether 'they' includes your husband or if he is just turning a blind eye to his relatives doing it. You need to be more assertive about making sure that your son does not get taken off for hours without you. Insist that you go where he goes and if he needs telling off you make sure you get in first and handle things your way - don't just allow other people to muscle in and undermine you.
Another thing that has not yet been answered is have you actually told your H that you are unhappy and regret moving, and what has been his response? Does he act lovingly towards you still, and does he even care that you seem unhappy? Obviously you need to be very careful about how you broach this, but what is your gut feeling about him now?
PLEASE just don't get pregnant again. Just don't. I'm not sure about SA but where I am in the ME we can buy the pill over the counter for about £5 per month. If you can get enough privacy and access to enough money to be able to buy it then DO. It will be £5 per month well spent no matter how broke you are. And if you could persuade your H to let you take it 'until you get back on your feet financially' then so much the better.
If the pill is really not an option then the pharmacies here do a HUGE range of vaginal douches with vinegar in them……sort of like an old fashioned version of the morning after pill.