Hi a warm welcome to the site. Being independent means you can use the wording of the study to draw up your own permission form and then align it with any policies & procedures you already have in place.
Consider who - name, how old - age/DOB, contact - address/email/telephone, how long the study will last - dates, where and how frequently observations will be made - times, what type of information will be recorded, who it will be shared with, where the information will be stored, if photographs are involved and why you want to undertake the child study - so you can comply with the data protection act.
You could also ask your tutor or CM network to see if they have any pro forma sheets?
Unit 3.15 Page 382+ of the work based EYE handbook on amazon.co.uk helps to explain the longitudinal study
Child study consent form or
permission child study google search
Hth xx