Depending on where the frozen food comes from, some can be as good or even better than fresh foods.
1. Frozen foods are usually the best of the crops. They pick the best (nicely ripe) to freeze as all frozen/packaged food have high QC levels.
2. Frozen foods if handled properly, do not lose much nutrients. Compare a frozen pea that is well kept versus a fresh pea that is manhandled with bruises, which would you choose?
3. Proper defrosting & reheating does not alter the nutritional content much as long as the frozen food is kept in proper containers. Always keep frozen foods in airtight bag or container to prevent contamination. Do not purchase frozen foods with damaged packages.
I do give my boy frozen food (peas & sometimes frozen strawberries). But it's not everyday & not every meal.