Thanks for your enquiry.
I am afraid you have to be 18 years of age to legally own a property, so you won't be able to do anything at this stage.
There is a possibility that you could put the property in trust for your Grandchildren, with just your name remaining on the Deeds, but this is slightly complicated, and you would need to take specialist advice on this. However, I would normally advise against this, bearing in mind the Grandchildrens young ages (if you set up a trust, you are legally committing yourself to giving up any ownership and power you have over the Flat, and amongst other things, you may lose any control you have if you felt in later years that you needed to sell up for whatever reason).
I would therefore suggest that probably making a Will, leaving the property to your Grandchildren would be a safer bet. Your local Wills Solicitor can of course discuss all the above with you at length.
I hope this assists and answers your question. If so, I would be grateful if you could rate my answer.