Many people who think they've paid over half their missionary child's support probably haven't. Remember, the IRS does NOT consider the $400 monthly payment to the church to be support. Health insurance premiums count as support, as does money you send directly to the missionary, but those likely don't add up to more than what the church spends to support your child. The church provides more than half your child's support, you do not.
The article I linked to earlier presents some ways to justify not counting the church's support as support, but even the author considers them a stretch. If you get audited you've got to get the IRS to agree with your interpretation.
My comments earlier today about the first and last year of your child's mission were too strong. You can count them as dependents in those "shoulder" years if you provide more than half their support, regardless of whether they are in school, assuming they don't make too much money. But again, be careful to consider the church's support when deciding whether you've met the 50% support threshold.
