Food poisoning from marshmallows is possible but very unlikely and very rare that a person gets food poisoning from eating old marshmallows.
Marshmallows are made from a mixture of sugar, water and gelatin that is then whipped to a squishy consistency to create the marshmallows.
Bacteria does have the ability to grow on old marshmallows but if they have been sealed in a package even when they are expired the marshmallows are usually safe to eat.
I've eaten expired marshmallows before and never had any health issues from it.
But there's a slim chance of food poisoning or sickness from eating old marshmallows but it's a very low risk.