Is this a charitable contribution?

Since you're talking about more than $250, you're going to need a letter from the church acknowledging any contribution. What you should do is tell the church that you're not going to seek reimbursement and would like them to acknowledge what you laid out as a voluntary contribution.

Now, regarding the two from your family, I don't see where the cost of a child's attending the conference would be charity, but chaperoning would be, so one or two of the tickets might not be a legit deduction.

DISCLAIMER: My opinion only.

First off, the disclaimer that I have no formal tax training.

But as for whether the money spent would be a charitable deduction, I think this ends up in the ever popular "it depends" category. Generally you can deduct out-of-pocket expenses that you (or presumably your family) have in engaging in charitable activities. I think that the important point here is to figure out who the primary beneficiary of attending the conference is. If the primary purpose of the conference is to advance the religious purposes of the church, then I think you can make a case for this being a deductible expense. If, on the other hand, the primary purpose of attending the conference is for the benefit of the people attending rather than the church and its purposes, it would not be deductible.

For example, I feel comfortable deducting the expenses of sending my kids on a mission trip to work in a soup kitchen, but don't deduct the costs of going skiing with the church youth group.

I would also check publication 526 to see if they have any examples that help illuminate this, although I suspect that there won't be anything exactly like this.