I have not been in the last two years but as far as I know there is no rule against bringing your own, nor any rules about searches.
A quick Google search in Dutch did not bring up any mention about there being rules about food.
And in the old days people used to bring their picnics, maybe some folks still do.
I read the whole site of the Keukenhof and did not see any mention of not being allowed to bring food while they clearly state what is not allowed in transports (no bikes, balls and that kind of things.) So I am sure that there being no mention of food at all in that respect means that the rules have not been changed since my visit.
You are welcome to use the restaurants they offer but allow you to eat your own food outside the restaurant areas.