Fr Chuck'sposting is accurate.
The custodial parent has the irrevocable right to claim the child (and get the EIC) unless the other filer can definitively prove that he/she provided half of the child's support.
Even if, for some reason, you cannot claim the child, if you provided a home for the child in 2005 and/0r 2006, you can file as Head of Household, which may result in some refund.
You do not refile. What you do is AMEND your 2005 tax return to change your filing status from Single to Head of Household. You may also want to amend your 2006 return as well.
Now, a better option may be to go to your in-laws and negotiate with them on this issue. You can amend your return in order to get the EIC, and they can amend their return as well. Both returns can be mailed in the SAME envelope with a cover sheet explaining the situation.
They will have to pay additional tax, plus interest (required by law), but the IRS will probably waive any penalties.