A Science Fair Exhibit is a vehicle for a child to display that child's idea about what that child has observed. All scientists use the skills and experience of others. Engineers use other people when building something so should your exhibitors.
Parents should always help where it is needed and teachers should not hesitate to help the child in any way either. BUT the study should be the child's and the conclusion reached be one that the child holds firmly.
Judges in science fairs will quickly sort out exhibits which are those of adults rather than pupils. Remember the child has to give proof of the investigation or solution, all the fancy lettering and sophisticated machinery will be of little value if a science fair judge realises that the child doesn't know what is being studied... if the child didn't in fact do the work displayed.
Primary school "classroom" exhibits will be accepted but not judged against individual entries.