WARNING: This is NOT a criticism, it's merely information!!!!
The actual, written policy is 'no sharing with people not on the dining plan'. This is generally considered to mean that you (not YOU you, but anybody on the DDP) cannot use their credits to buy food for non-DDPers.
As stated above, once you have your food it's unlikely any Cast Member is going to care who eats what* BUT!!! There have been occasional reports of a restaurant or two sticking to the exact wording of the terms and conditions of the DDP, and not allowing sharing period. It's likely NOT going to happen to you, but be prepared in case it does.
Also, if you let the restaurants know ahead of time that you have dietary restrictions, they will VERY likely be able to provide an alternative (aka one you can eat) dessert.
*Of course, if there are four of you and you only order two meals, period, it's considerate to tip 'extra', or in the case of the DDP, to tip above what the plan covers.
I'm sure you've heard the old saying, "sometimes it's better to ask forgiveness than permission." Sharing items of food (i.e., parts of a meal) that you've legitimately ordered with your own credits on the DDP with someone not on the DDP is IMHO one of those situations. WDW would be a most unmagical place if you could not do whatever you wanted with your actual food once you got it within the terms of the plan. In fact, one could make a sound legal argument that once the food is delivered to you in exchange for your credit, you have the
right to give it to anyone you wish, even someone (gasp!) not on the DDP. And yet, as kaytieeldr has pointed out, Disney's quasi-official* stance is "no sharing between DDP'ers and non-DDP'ers," and since whomever at Disney you ask a question about "sharing" (be it a restaurant CM, dining CM, Guest Communications CM or Guest Services CM) is unlikely to be a lawyer, you may get the stock "no sharing" response even if all you want to do is let your granddaughter eat your dessert, which I am 100% positive would never be an issue if you didn't ask about it in advance. So, OP, just do what you planned, don't unnecessarily complicate things by asking in advance, and in the EXTREMELY unlikely event that a CM takes it upon him/herself to question why your granddaughter is eating your dessert, politely ask forgiveness.
* I say
quasi-official because the official language actually says nothing about sharing
items of food, it's actually about sharing
entire meals, or in other words, what you are entitled to get in exchange for credits. The official quote from the DDP brochure is:
Q: Can I use my Disney Dining Plan to redeem meals for friends or family members who are not included in my reservation and have not purchased a Magic Your Way Package Plus Dining?
A: Sorry, you may not use your Disney Dining Plan to redeem meals for friends or family members who are not included in your reservation and have not purchased a Magic Your Way Package Plus Dining. [Emphases added.]
I feel 100% confident in saying that what Disney is
really concerned with, and what the quoted language is meant to address, is the proverbial situation where there are 4 people at a table, 2 who are on the DDP and 2 who are not, and the 2 DDP'ers try to use 4 credits to pay for everyone's meals. That's why there is software in place that prevents the redemption of more adult/kid credits at one time than there are adults/kids on the room reservation. But I do not think it is an actual violation of any official DDP term or condition to share bites of or even give an item of food away once you've gotten it legitimately with a DDP credit.