While this is usually a service they provide, you may want to make alternate plans.
There have been some challenges recently with Delta fulfilling meal services due to the meal service providers requesting payment upfront, etc. This probably will be resolved by time you travel,however.
In today's age, it's always good to get on a flight with at least half-full stomach. We were surprised on our Skyservice flight recently from YYZ
to get a 'snack pack', which could have maybe filled a 2 year olds stomach.