7 year old British born from Mauritian parents intend to stay in Mauritius for one year, do we need a visa ?

No, you don't. However you will need to acquire a Mauritian Identity Card when you get here. You will need the following documents to do that:

• Birth certificate of your parent (either mum or dad)
• Identity Card of your parent if they have; or their passport
• Your birth certificate

This will entitle you to Mauritian Citizenship and all restrictions will be removed when you come to visit. It will also save you a lot of hassle in the future because everywhere you go, all they ask for is your identity card. So the sooner you do it, the better.

Update : Just realised you said 7 year old. So forget what I said above, it only applies to adults. Your child will not face any problems because you're a Mauritian parent.

Someone who is Mauritian by birth or descent does not require a visa.