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he's probably purposely keeping it from kenji so he doesnt try and think that his father's some mass murderer and try to learn his secrets if kenji thinks that's sooo cool and like most boys, probably will

depending on which storyline kenji learns that he was battousai from depends on how he'll react if it's from the seihousen storyline (which is a piece of crap) kenji will most likely hate him even more for

1. not telling him in the first place
2. hiding something from him
3. he already hates him and it'll just add up

if it's something like from the manga which is the truer story then he'll probably be mostly accepting of it, and hopefully be more like yahiko

hope i answered accurately heh O.o

Hmm, tough question! (At first I thought you were asking about Kenshin's dad... I was like, o.O we know stuff about him?!?!) I think Kenshin would probably keep that information for Kenji for a good long time, unless something happened and he had to reveal it. His reasoning would be that he'd want Kenji to be mature and sure of his beliefs before he's presented with the way his father once lived opposite of his own beliefs. (Though, it's my opinion that Kenshin never deviated from his core beliefs, he just found two drastically different ways of pursuing them.)

But, Watsuki did right a short sequel story to Ruroken about Kenji and Yahiko's son Shinya, I believe. They fought over Kenshin's sakabatou, which Kenshin had passed on to Yahiko and then onto Shinya. Kenji had sort of taken a dark road in his life and acted hitokiri-ish, except without his father's pure motives. That made me sad to see Kenshin's son act so differently from his father, but it did imply that Kenji already knew and had known for some time about his father's past. (Albeit, he was rather grown-up by this time anyway.)

I also seem to remember that part of Seisouhen, the part about Kenji going off and studing with Hiko, is also canon, which would imply he knew at least part of Kenshin's history by age 13. But on that I'm really not sure - I could be remembering entirely wrongly. It's been a long time since I read any of those subsequent shorts.

hmmm... Kenji would most likely find out but I doubt any sane parent would tell their children about something like that before they are at least a teenager.

It's kinda sad how, according to what Watsuki said in the last manga, Kenji becomes "more cynical than Saitou and more conceited than Shishio." It's hard to see the son of Kenshin and Kaoru being like that...

where did you hear about him going into the army?

He might of heard all of his dads story's and tried to mimic some of the characters in them, like wasuki-san said, he never saw a hiten mitsurugi-ryu move but he mastered the swordstyle just from hearing about it.

Well, Kenshin would certainly try his best to keep Kenji from becoming like that, but in the end would understand that he can only serve as a guide. Ultimately, Kenji's life is his own to live as he sees fit.