Phy on the login cookie that my system keeps eating.
Excuse the brevity. This is my third attempt on an important subject.
I have taught Latin to homeschoolers for 6 years. I worked for a major Latin textbook publisher for 5. I'm ignoring the Greek part of the question, since I just don't know as much about what's available for elementary students.
There are three ways to teach Latin in my opinion. 1. Traditional Grammar/Translation. 2. Reading with English explanations. 3. Full on immersion. I've indicated the grades I think each works best with.
For the grammar/translation approach, Latin for Children by Classical Academic Press is the best. Far and away. Affordable. Lots of support material. Large user base. Not a full Latin course, but a good enough starting point. Grade 3-6 (maybe 7-8)
For the reading with English approach (and I think the books are generally of a much higher quality in this group):
1. Minimus and Minimus Secundus. Really solid and will appeal to children. Not a full Latin course, but lots of great exposure to the basics. Also presents mythology and daily life in Roman Britain. Grade 3-6
2. Artes Latinae. I like it quite a bit, but I'm perhaps too familiar with it. The biggest drawback is that students will need to work on the computer separately. Grade 5-12
3. Latin in the Christian Trivium. Probably more of a grammar/reading hybrid. Very Christian. So if that's not you, this may not be for you. Grade 6-12
4. I'm working on something for the homeschool consortium I work with. It's not quite ready for prime time, but if you're curious, drop a PM.
Immersion: Lingua Latina per se Illustrata. I can't say enough good, but do bring the right attitude to the book. It is completely in Latin, but it cleverly written so that everything can be understood on its own (or should be).