Using Latin to teach grammar

[Ann in OK]

I just read an article in Practical Homeschooling by Martin Cothran in which he recommends using Latin to teach grammar. Did I read this correctly? How is writing incorporated with this? I assume the writing would be where one would teach mechanics. DH, who has taken Latin, thinks this is a great idea. Also, dd will be in 4th grade next year when we will start this. Should we just start with Latina Christiana or will she not be too old for Prima Latina?

TIA, Ann