Depending on how you feel about it, you could perhaps have your child Baptised/Christened or have a naming day (if you are into that) and have the Priest/Celebrant on hand to marry you at the same time that way it really is a surprise.

Or wait another 6 months and have the child's first birthday (inviting you family and friends of course) and have a celebrant rock up and marry you then.

Its not a surprise if you tell people you're eloping, and some people might not come to the party if thats the direction you go down.

HTH Nae x