Sorry you are going through this excitedmtb.
I am mum to a little boy who joined us by adoption and a birth dd.
I am sorry I can't really advise from experience as my family have pretty much done what I asked!
I am very lucky.
Were I in your shoes I would take a leaf from Pelican's book.
I would be less tolerant of this type of behaviour. Your relatives are manipulating you, possibly innocently, and they need to be stopped.
personally for a limited period of time I would pick up family on any times they attempt to interfere in your parenting. I would not be nasty about it but I would try and deal with it there and then when it happens and be very accurate. What you said, what they did, how this has affected DD. Then if it happened again I would pick it up again at the time and by email with a statement like “This is making family occasions difficult for me, because I do need to get dd into a routine, to understand etc etc”
So for example, the cake, if I could get to my ds before he was given the cake or before he started eating it I would remove it and say to the relative, I said no more cake it is bad for his teeth.” Or whatever, then add some compliment to ds “You have such beautiful teeth." Then tell ds he could have something nice to eat later (if you want to) and/or suggest something fun to do now. My ds does like his sweet things and often takes it very personally he cannot have chocolate in the middle of the day and at the end of the day! I do talk about teeth a lot because I do not want to talk about weight or ‘fat’ etc.
If my ds had already started eating it I would possibly take it away, just depends how I felt. That would probably be a very hard call but if you thought for example the cake might make him/her feel sick or ill (possible if he/she had already had cake a few minutes before), I would. If the child started crying I would comfort them and would explain to the person who had taken it upon themselves to give my child cake that the tears were not because 'I' had refused cake but because the other person had given it.
I know that sounds dramatic and you don't want to be the bad guy here, to your child or the relatives but they do need to understand how their under mining you is causing additional stress for your little one and you.
I hope you told them your child was upset by what they allowed her to watch on the Internet. What did they say? It is not too late to tell them by email if you have not. Especially if you dd has mentioned it again since then.
You said I don't wish to alienate family and would rather have their support. Do tell them that if you have not already told them. That you value them but ultimately your child must come first.
Personally, I would speak to the relevant relative by email to explain how unhelpful their behaviour is, and I would throw in how sad you are because you were enjoying seeing them, lovely food, fun day etc etc, such a shame it happened but look on the bright side and remind them they can help you to establish good routines for dd by backing you up!
I know that all sounds harsh but I think you need to spell it out.
Why do you think they will take literature better than words from you? If you do want to give them something to read I would print off these…. (if they say what you want to say) …
www.parentdish.co.uk/mum/10-things-adoptive-parents-wish-their-friends-and-family-understood/
www.adoptionuk.org/resources/article/coping-christmas
I did look a lot and could find not much at all. You could try adoption UK and BAAF.
I agree you do not need to give them any examples of why this is necessary that are specific to your child. You can talk in general terms. If they cannot understand this just limit your time with them. Meet with them in neutral places, park, zoo, soft play etc. That way you can leave early if you need to and you don’t have an issue with throwing them out.
I would also use divide and conquer, speak to them each separately, assuming the best "I know you will want to help me." and "I know you will understand.” Even though they seem not to. Expect the best of them and show that you are disappointed when they behave badly but that things can be better.
Hope for the best but if you don’t get it, limit their time! IMHO
Good luck.
Oh sorry, long! Just thoughts!