I'm not sure that you could say it in french?. Halloween is not a traditional french celebration and was imported for commercial reasons about 5 years ago. The allsaints day that comes after is closer to the Celtic Samaïn (day for talking to your deceased ancestors) when people go out to put flowers on their familly graves mostly crysanthnems (DO NOT give a French person crysanthnems).
But otherwise try sac de Halloween for want of a better way
Joyeaux Halloween!
un cartable
sac a dos
dans mon sac
sac de bonbons
"Sac" in French is spelled as "s-a-c."
Easy : Halloween = Halloween
Halloween
You can buy a regular or creative Halloween bag at Walmart, or your local Halloween store. You can even use a plastic bag. If you want to choose how you want your style to be you can always make your own at home.
Rocks
Halloween in French is L'halloween. Pronouced L'olloween
You can buy a halloween gift bag at many local halloween stores. You can also find them available for purchase at local parties stores. Grocery stores also have them available.