I don't know about teaching children when they're very young, but I do know that when I was in early highschool I was studying Japanese and Indonesian at the same time.
In an English class, I wrote a 2000 word essay in Hiragana, using Indonesian language. The teacher thought I was taking the mickey and yelled at me for it. Then she took it to my Japanese teacher who translated it to Indonesian, and the Indonesian teacher who translated it to English.
I got a B for that essay at the end of it. I think the English teacher saw the funny side by the end of it when she realised I hadn't done it intentionally.
My point is that if a teenager with a pretty good grasp of language can have their brain broken enough to make a mistake that big and not notice it while they're doing it, I'm sure a toddler would have just as much if not MORE trouble! So I wouldn't be suprised if it took a trilingual child longer to speak.