You want to talk to the family in regards to noticing their baby is having a reaction to food? The nanny should be doing this, after all, she is the caregiver. However, if you’re truly concerned, then yes, say something but are you sure the child is reacting to the food(s) and nothing else?

Of course it's not your business what someone else feeds their child. How could you possibly think otherwise?

Leave it to the other family to manage.

https://www.mayoclinic.org/healthy-lifestyle/infant-and-toddler-health/in-depth/healthy-baby/art-20046200
https://www.webmd.com/parenting/baby/baby-food-timeline
https://pediatrics.aappublications.org/content/122/Supplement_2/S36
https://www.aap.org/en-us/advocacy-and-policy/aap-health-initiatives/HALF-Implementation-Guide/Age-Specific-Content/Pages/Infant-Food-and-Feeding.aspx

There’s conflicting advice. Start cereal between 4 and 6 months, no wait and only breast milk til 6 months. Introduce one at a time, no introduce combinations. Wait 3-5 days, or is it wait a few days between new foods, or maybe don’t wait?

It’s absolutely not your business what the other baby eats!!

Those are old guidelines, OP. There is no need to do the three day introduction.

The whole waiting three day introduction thing has been debunked. It was useless. Your baby can have anything except honey.

We're in a nanny share with one other family, where there are two babies a few weeks apart in age, both currently about 6-7 months old now. We've been doing food introductions per the AAP guidelines, such as one food at a time at least 3-5 days in a row to ensure no reaction and then follow-up exposure sporadically. The other family started that way, but now is just dropping baby off with prepackaged kits with multiple new foods included, which the baby is starting to show mild reactions to. I'm not worried about my baby's exposure, as we have had great success with our introductions and the ingredients in their packaged foods are all things we've successfully tested, but I'm wondering whether I need to address this with the other family? Is this my business, the nanny's, or just the other family's? Thoughts?