I guess "officially" they have the option to ask you to dump it but on our daytrips I've never had a problem bringing tons of snacks (packaged in ziplocs, Lunchables, sandwiches, fruit in plastic containers...) and drinks (juice boxes, water [which I don't bother lugging anymore as you can get free ice water anywhere they sell ice/cup soft drinks!], sodas...)
Last week I took a giant foil insulated bag with no issue. Often I'll grab a $1 McDonald's soft drink on the way in and that's always been allowed as well.