We arrive in Paris early morning and will need to wait for our train to Brussels so I thought we might do our duty free before we leave the airport. Is this possible?
It won't be possible. At duty free shops, you have to present your boarding pass for your departing flight outside of the EU. If you can't prove you're leaving the EU, they will charge you tax/duty.
Thanks
Also, duty free are behind security so depending on how you exist the plane, go through immigrations, etc. you are most likely outside of security and it would take a boarding pass to reenter the secure area.