How to handle severe dietary restrictions?

I don't think it's rude, but it might be better to call her? You can ask if she prefers to bring her own food for her daughter or if she'd like to speak with the caterer.

That's what I was thinking. Reach out to the mother and ask her to call the caterer as she's more equipped to represent her daughter's allergies. I feel like a note is very impersonal and could be overlooked.

Agreed. I'd find this off-putting. A personal phone call would be much better.

I'd wait until after they RSVP before bringing it up with them. Some people might be put off by this and decline so they're not a hassle to you. If they RSVP yes, follow it up with a call asking them if they're happy for you to just pass that information on to the caterer or if they'd feel more comfortable dealing with them directly

We have one person coming who can literally eat like 5 things. I'm not exaggerating. She offered to bring her own but it is important to us to host all of our guests, so we are having a specially made meal for her that will meet all her requirements.

I would not leave a note. I would talk to the mother first and ask either for something in writing to give the caterer or offer the contact information if she would prefer that.

We had a number of nut and peanut allergies, and rather than risk it, we asked the caterers not to include any nuts in the food. They happily obliged. There were a couple of seafood allergies and some vegetarians, so we asked that dishes containing meat and seafood be labeled. A couple guests, including my husband's parents, were on very strict diets- vegan, gluten free, no sugar. I made sure they understood and they basically at a big meal before the wedding. They had a great time and no one passed out, so it worked out fine.

I would include a handwritten note saying something like "We are aware that [daughter's name] has food allergies and would like to make sure our caterer has the necessary accurate information. If you would prefer to speak with the caterer directly, you may contact them at ____."

Your guests will appreciate that you are taking their daughter's allergies seriously and that you made the effort to address the situation. If you don't ask, they might not be able to fit everything clearly on the card or confirm specific details, and they might decide to just play it safe and bring their own food for her without checking with you about details first. I have dietary restrictions, and if a wedding invitation doesn't address the issue clearly I just assume that I will need to eat a big snack before the reception, because I would feel uncomfortable bothering the hosts to ask about food. Anyone whose child has severe food allergies is used to navigating these weird situations and has probably made plenty of phone calls to restaurants inquiring about allergy issues, so I doubt your relatives will mind stepping in to make sure everything is handled correctly.