I packed my daughter lunch when she started daycare. They have since me stopped serving lunch because of allergies/restrictions etc so it has worked out for me.
Ask to have a copy of their menu. Highlight items she can have and send substitutes for the items she can't/or you would prefer she didn't eat.
I work in the Center my children have grown up in and we have a very strict snack menu; no juice, low sugar, high fibre. We serve fresh fruit/veggies with every snack(am and pm). Hopefully the centres menu will surprise you