Hi leesmum,
If you come and meet posters on the Trip Advisor Disneyland Paris / Marne La Vallee forum you will find many who have been in October for Halloween festival, as this is very popular.
There is a long list of Top Questions with answers on both forums ( which are linked ) and you can also read through previous questions or ask your own there. Posters who know DLP very well are glad to help.
Disney do allow some drinks and snacks to be taken into the parks through security checks, but not large picnics. You will notice people making up rolls etc from food at the breakfast buffet in the Cheyenne.
If you have not already ordered a birthday cake for your son's birthday Character meal, you can do this for ? 25. They are quite large, but Cast Members ( staff ) will bring the lit cake to your table and sing Happy Birthday, along with any Characters present at the time- maybe Mickey and / or Minnie.
Which Character restaurant have you booked? Have you made a reservation?
October half term is very busy, so I definitely recommend that you make advance dining reservations for your stay before you go.
French schools are on holiday 22nd october to 3rd November.
Eating in the parks, Village or hotels depends on whether you want fast food, buffets or table service. There are a few options for Character dining, some more expensive than others.
HB vouchers can be used at lunchtime as well as dinner, so I would not discount booking it just because you want a Character dinner. Cafe Mickey is HB Plus category, so Plus vouchers can be used here to cover the meal. People with Standard HB vouchers can still use them in Plus or Premium restaurants, but they are taken at face value like cash, and deducted from the bill. You don't get the soft drink included using the voucher in this way though.
Which HB category to book depends on which restaurants you wish to use.
You can see restaurants menus and prices on dlrpmagic.com -they are usually pretty up to date. It is a very good website for DLP planning in general.
www.dlrpmagic.com/planning/dining/restaurant-menus/