Firstly, as a Sunday school teacher I can tell you it has changed!!! We now play games, do arts and crafts, sing songs and also teach. We teach about love, caring and forgiveness I have no problem with teaching a child, ANY CHILD that.
As most of the children in most non-denominational schools will celebrate Christmas and Easter, even if it is just to recieve presents or chocolate eggs, where is the harm in teaching them one of the reasons why these traditions have come about.
As for Harvest Festival, these days it is all about collecting for the poor and needy, the fact that the church or the Salvation army are the bodies that organise collection and distribution of donations means that of course they play a role... mainly in thanking the children for their generosity.
The National curriculum states that all children recieve RE lessons, they will learn about Hinduism (and probably celbrate Diwali) Islam (and will visit a Mosque and celebrate Eid) Judaism (and will visit a Synagogue and make oil lamps or dreydls) as well as Christianity.