Like ocid, I've already heard des bonbons ou un sort.
However, to complete what micia said, my personal feeling is that Halloween is getting less and less anchored in French traditions.
Ten years ago, when I was a kid, many people were displaying some pumpkins and would decorate their houses.
That's when Halloween came in France and it was quite popular. I remember some years when my doorbell rang 20 times.
But during the last 5-7 years, I think that the trend has been declining and keeps doing so. This year, I've seen 4 groups of children in total: two of them were not dressed up at all, and another one came on the wrong day (30th October)...
Anyway, that's only my personal feeling about Halloween in French culture.
But still, to come back on topic, I'd personally say des bonbons ou un sort, that's what I've heard.