there is a web site
http://www.ukdps.co.uk/Have a wee look at that found the following on it.
A child's legal name can easily be changed by Deed Poll providing the applicant has parental responsibility for the child and anyone else with parental responsibility also consents to the name change. If your child is 16 years of age or over, they must apply for their own Deed Poll for which parental consent is not required.
There is obviously more information on the web page, but i think the only way the name can be changed is if every one with parental responsibility agrees.
Not sure if the web page i have found will be of any help,
Not easy when someone changes childs name, personaly im not sure if in all situations this is the right thing to do. Can be confusing at school etc when parent and child have different surnames (speaking from my own personal experiance as a child)