C.C.
I would say something like: "Please join us for a picnic at XYZ Park. We will be celebrating Joseph's Christening. A barbeque lunch will be served, with cake and ice cream afterward." Or something like that. That way people know that your picnic is to celebrate a specific event and can bring gifts if they like.
Is there a reason you would not also invite them to the christening? Even if they are of a different religion, most people do enjoy going to these types of events (since the guest of honor is so darned cute!). Then you could issue the invitation to the christening with "picnic to follow at XYZ Park."