MFL rather than Latin at that age. If you do go on to a classical language, I've found Greek to be more 'useful' than Latin, but both if possible.
Really, as others have said, secondary language acquisition is best facilitated by mirroring the process of primary language acquisition. So, whatever MFL you speak best, regardless of its 'usefulness', will be the best one to expose your dc to, with plenty of practice at home. And books, dvds, holidays etc.
When I was a MFL teacher (before being made redundant when languages were made non-compulsory for GCSE), there was a large scale study which came out (around 2008?) showing that children who had learned no MFL in primary and started as beginners in Year 7 were of an equal standard with children who had done the same MFL from Reception by the time they got to the end of Year 8. Basically, learning an MFL just in a 30 min/hour lesson a week might help your dc in some non-measurable way, but they won't actually acquire any sort of fluency in the language unless there is long and frequent exposure to the language in a natural setting.
By the way, the ease of picking up a Romance language after studying Latin, which you mention, works both ways. Having studied a Romance MFL will help them to pick up Latin later on.