If we signed over our property to a child and have a life estate, can we undo it if all parties agree?

I would seek help from an estate planning / real estate attorney in your area.  I assume that you already filed the deed listing you as having a life estate.  The "owner" of a life estate technically has no ownership interest in the real property.  But the remainderman (the person who gets the property when you die) only has a future interest.  The grantor - which is also you - has the ownership interest.  So you need to check with the attorney if you have the right to revoke the interest you gave yourself and plan differently for the future of the property.  If you all agree I would assume that it will be fine but I caution you to have a written agreement drawn up with the signature of everyone who is effected by the revoking of the life estate.  Good luck.   

For a definitive answer, you need to consult with an attorney who can review the situation and the documentation; every case presents its own issues and complications.

That said, as a general proposition or principal: anything that 2 or more people agree to do, they can then agree to undo; the law does not make contracts, agreements, transfers, etc. irrevocable when the parties themselves wish to undo them. The complexity can come from 1) how to make sure the transaction is properly rescinded or unwound, to put everyone back in the same position; and 2) any tax consequences, since property or asset transfers have tax implications. Therefore, the short answer is that if everyone wants to do this, you should be able to; but the doing of it may be complex and could impact other areas, like taxes, so you should have an attorney's advice and assistance. Good luck.