"Mom, what are we eating today?"
Translation:Mamá, ¿qué comemos hoy?
In Spanish, when you use the 'ing' form, you must be doing the action at the time. The 'ing' form is much more limited than in English. In English, 'I am working' could mean you currently hold a job, or that you are at the exact time actually working. In Spanish, 'estoy trabajando' can only mean that you are actually working at that moment.
I think it's how English uses "to be"/are differently. In English you can't say "what we eat today?" You need "are we eating" to get the right verb tense to agree with "we". But comemos already covers the tense and the "we", so no form of "are" is necessary in this sentence in Spanish