Is there any risk of problems if my child was given an extra dose of dtap vaccine?

No. Research has amply proven that an extra dose of this vaccine causes no harm and may provide stronger immunity.
No. This vaccine has been remarkable since it was modified & re-released in ~1992. The frequency of any side effects like fever, crying spells etc went from expected to almost non-existent. Other than feel sore at the site, the kid should have no issues with the added dose but may have a little better immunity.