A sister asked a similar question with regards to attending a mixed Eid party with family:
It is encouraging to note that you are trying your best to refrain from haram and adhere to the laws of Sharī'ah.
Islam is a religion that promotes modesty and self respect; the Holy Prophet (sallallahu alaihi wa sallam) said:
«إِنَّ لِكُلِّ دِينٍ خُلُقًا، وَإِنَّ خُلُقَ الْإِسْلَامِ الْحَيَاءُ»
“Verily in every religion there is morality, the morality of Islam is modesty.” [Sunan Ibn Majah, 4182, Book of Zuhd, Chapter of Hayā]
In another hadith, the Holy Prophet (sallallahu alaihi wa sallam) states:
«الْحَيَاءُ مِنَ الْإِيمَانِ، وَالْإِيمَانُ فِي الْجَنَّةِ»
“Modesty is part of faith, and faith will be in Paradise.” [Sunan Ibn Majah, 4184, Book of Zuhd, Chapter of Hayā]
Interaction with ghairmahrams and mixed gatherings disguised under Islamic names are not only contrary to the teachings of the Quran and Sunnah, but they also corrode the fine natural values Islam attempts to inculcate within us. As a result, these gatherings also become a reason for invoking the anger of Allah Ta'ālā.
If the gathering in question does not exercise hijab and allows men and women to freely intermingle with one another, then it is imperative for you to avoid attending such a party and advise your husband to do the same. You should excuse yourself from attending this gathering and kindly inform your family members about your stance concerning this matter.
May Allah Ta'ālā keep us away from fitnah and grant us success in this life and the hereafter. Āmeen.
So for you to keep away from mixed gatherings shows you have concern over your imaan and therefor you are not being "extreme" at all. What difference is there between a mixed event like this and the market places (high streets and shopping centres) where one should only go if necessary. Mixed gatherings are the most hated places by Allah as they are places of fitnah and where shaythans gather.
However if the gathering were sfully segregated between women and children and men, then the ruling would be quite different as long as other aspects of shariah were not compromised.
So you are absolutely right to keep away from mixed events even under the guise of "Eid" or any other religiously affiliated event. We must realise that Allah only wants the best for us and Islam is about "prevention is better than the cure".
Islam does enjoin that we enjoy our lives but within the boundaries of our deen. By doing so we will have peace, blessings and contentment in whatever we do but by transgressing the limits we are putting ourselves in danger and we will not feel at peace within ourselves.
And Allah knows best in all matters