Fidelity offers the ability to 'designate' accounts. This means you can hold an account in an adult's name, but give it a label such as the child's initials or name to identify which assets are the child's.
For example, you can complete the online forms in your own name, but write the name of the child in the section headed 'Designation'.
Please note that designation may not be considered by HM Revenue and Customs as conclusive evidence that the assets have been given to the child.
This is a complex area of tax planning and you may need to seek guidance from a financial adviser.