We are staying in Paris for a few days and then taking the train to Belgium. 8:55 AM Train - Gare du Nord station (leaving from) on Thalys Hotel is by Porte Maillot Metro and RER. We are planning on taking the Metro to Gare de Nord, but when should we leave the hotel and what's the best route on the Metro (I know we'll have to change lines, that's not a problem)?
Take Metro line 1 from Porte Maillot to Chatelet. Transfer to Metro line 4 and take it to Gare du Nord. Allow 30 minutes for the entire ride.
Since you will be traveling about Paris for a few days and using the Metro, why not take a dry run to Gare du Nord from your hotel, following the above route. Doing so should allow you to experience the rhythm and guideposts of the metro. I love the Paris metro and still think it's the best designed of any I've ridden. Best wishes.