Mixing of men and women leads to bad consequences and negative effects, whether that is at wedding parties, at work or in school. This has been discussed in the answer to question no. 1200. 

Having each family sitting at a separate table does not prevent this mixing when people come in and go out, as it does not prevent them looking at one another and so on, especially if there are among these families people who do not observe proper hijab, and evil consequences occur as a result.  

Moreover, if each family sits by itself, that does not achieve the purpose of celebration and spreading happiness and joy, and letting people get to know one another and make friends with one another. 

Hence we advise you to hold the celebration as you initially suggested, in a place where men and women can be separated. This is more fearing of Allaah and is more likely to achieve the aims of the gathering. 

We ask Allaah to bless you and to bring you together in goodness, obedience and righteousness. 

And Allaah knows best.