I think only if both donor and child wish to. If agreement is initially made for no involvement then future enquiries can be made later to see if parties have changed their minds, with a respect for the other parties right of refusal.
A child will always be curious to see their biological father, however the privacy of the donor should be respected. Every situation is different and it all depends on the original agreement. Maybe a website could be setup that reunites children with their donors, therefore only if both parties join can they get in contact with each other - kind of like a friends reunited.