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British Nationality
Apply for a British passport.
Assuming that you are their FATHER, they have been British since they were born. Children born in Belgium generally acquire the nationality of their father.
Your children don't need to claim British nationality, they just apply for a British passport, although the initial application won't be either quick or cheap these days and if they have acquired Belgian nationality at some time I would respectfully suggest that they don't bother. Unless you were employed in some role that represented the British state when they were born, they won't be able to pass British nationality to their children.
Not sure if this helps you. Probably worth a read.
https://www.gov.uk/browse/citizenship/citizenship
I contacted the British Embassy in Brussels a few years ago for help with a last name issue when we registered. They were very helpful. Might be worth contacting them for advice!
Start here:
You are the mother, a British citizen born in the UK, and the father is a Belgian citizen, therefore your children already have dual-citizenship.
They can apply for British passports at any time but it is a real, and expensive, hassle applying when resident outside the UK (even for renewals, let alone first issue), and it is difficult to think of any benefits it would bring them that a Belgian passport wouldn't.
One possible, and relatively cheap, safeguard for the future might be to get each of them an official copy of your birth certificate which you can apply for online to the registry office of the place where you were born. For the purposes of British administration, these official copies don't have any "only valid until" date unlike Belgian ones.
