Hi,
This thread really exploded!
I'm a Muslim and I think you are doing a great job being supportive especially as you are an Atheist. Thank you for supporting your child.
I echo many of the other advice.
There's no "real" Eid foods just something a little more special than the day to day foods.
I'm from a South Asian Background but one Eid I just make the favourite foods of the family member which varies from South Asian cuisine to Lebanese to Mexican.
I didn't read all the messages so I'm not sure if you replied with where you are based but if you need to rely on premade stuff- then the deli sections of supermarkets are a good starting point, you could grab things like hummus, taziki, marinated olives and falafel, and you can make a basic fatoosh or a tabboule salad (lots of recipes online) and some pitta bread and put this together as "mezze platter" or a charcuterie board. This is relatively low effort and looks amazing.
You could also make some puff pastries with cheese and spinach stuffing.
You could get some dates and stuff with with some nuts or nut butter and covered them in chocolate.
Or you can buy ready made rice pudding and plate it in a bowl and top it up with some crushed pistachios and or pomegranate for an added flare.
Or if you can bake then maybe you could do a Persian Love cake? https://www.recipetineats.com/persian-love-cake/
For a main you could do rice with lentils (https://www.themediterraneandish.com/mujadara-lentils-and-rice-with-crispy-onions/#wprm-recipe-container-10503) I use tinned green lentils for this as it saves me time. And this dish is packed with flavour and no one will miss the meat/chicken if you are not able to get hold of some.
Most supermarkets also sells frozen vegetarian samosas also!
Like you already identified, the 'halal' aspects of the food is buy having halal meat/poultry, no pork and no alcohol. Other things like no animal fats (like duck fats/ goose fats) are also not allowed unless they are halal certified.
We also do things like rack of lamb, roast potatoes, chicken skewers, bean tacos, hummus, burgers, chickpea patties, chicken wings.
So if your daughter has specific favourite foods that she used to eat then you could absolutely make those and just ensure the halal aspects are followed.
Amazon sells cute Eid decors if you wanted to get some.
I think your daughter really just appreciates that you are accepting of her choice and want to be there for her as reverts often have a very difficult experience from their social circles due to reverting.