Out of nosiness interest @Ilovepugs2017 how did you meet this friend 2 years ago, and why/how have you become such close friends in such a short time-frame - especially as you say you have only actually met in the flesh "a handful of times"?
I am interested in this aspect because if you weren't such "close friends" it would surely be easier to just let the friendship quietly slip away, rather than either of you giving each other rules on your behaviours within the friendship.
To me, the age difference between your DD and her siblings does make a legitimate reason for the difference in treatment given by your friend. A 5 year old is still very young, and I think it is a natural instinct in many of us, to want to give the "baby" of the family lots of little treats, and cute toys and clothes for 5 year olds will catch our eyes (and imagination much more than the equivalent for
10 + aged children.
If your DD had been a "later in life" addition to the family, and her siblings were 20 and 22, would you still expect your DFriend to buy treats for them as well, and if not, at what age would you consider that it was alright to not include your older children? In other words, what age is your cut-off line?
I think that if I had ever been in your situation, I would have just smiled at my youngest getting little presents, and said to the other two (if they noticed anyway) well that's the perks of being only 5, whereas you two have the perks of staying up later, watching more interesting films, getting to make a lot more choices, not having to be in the same room as us when my friends come to visit, you two can go on sleepovers with your friends (if that is something they do) etc etc etc!
I am a great believer in equality and fairness, but I am also a big believer in children having more independence the older they get, and them learning about some of the "not too harsh" realities of life, and that outside of the family the older they get, the less interested parents friends will have in them - these are just a few examples. They will soon not even be at home when your friend visits OP, and it is human nature for adults to become less interested in teenage children, and then the adult children of their friends and acquaintances, unless we are talking about long-held friendships of the parents, where each child has been known since birth.
In my opinion you are making this a far too serious problem OP, when you should just be teaching your children to laugh it off. I am actually embarrassed for you that you told your friend that every time she gets your 5 year old a present, or a treat, she has to do the same for the (quite a bit) older ones. To me that is very bad manners on your part. However, I do hope that you are right about your friend taking it well, and understanding your slightly ott reasons, and that it wasn't just her initial reaction to you until what you said sinks in.
Please try and relax a little OP, or through bitter experience, I can warn you that it is going to be a long and very tiring life. I wish I hadn't been brought up to expect to give and receive perfection - it is impossible, and again, imo, not even desirable.