Due to grinding and jaw being slightly out- of- line, the dentist has recommended a custom night guard for my 6year old. Is the guard a choking hazard?

Role. The purpose of primary baby teeth is to hold space for the adult permanent successors, so if there is severe wear and attrition on baby teeth, and as long as the roots are there to hold space, it's generally okay but speak to your orthodontist as well.. Custom night guards not going to fit after baby teeth loosen/come out.
Grinding is common. Grinding is common in children. Generally custom night guards ar not made for 6 year old children as they still have primary teeth and the jaw is still growing.
Night Guard ? Your 6 years old will never wear a night guard. Bite out of line? Wait until he grows up, perhaps he will need ortho.
Grinding is normal. Children grinding their teeth is quite normal, and there are usually no TMJ symptoms. Since your six year old is growing and changing teeth, the nightguard would only fit well for a very, very short time. May i suggest you get a second opinion from a local pediatric dentist who can examine the child.