if you cant afford the field trip, keep them home.
If you are not going to give your child money then let them know. Make sure they are aware they can't buy anything. Tell them it is rude to beg for money or items from others.
If you can't afford to pack them lunch then keep them home. Do not expect others to give your child their lunch or for the chaperone to buy them lunch. It is rude and irresponsible.