It’s part of the Bois de Bologne in Neuilly. By taking the #43 bus from Gare St, you can get there directly. Lazare and then walk tout droit/straight ahead. Do not wander into the boulangerie on the corner for an ice cream as I did. The woman gave me the wrong directions. Once you’ve entered the gardens the Bagatelle is down on your left. Really you can’t miss it though I’ve managed to with all my previous visits to Paris. My big mistake.