Great question... Children are santified by a believing parent. However, if any teens, in my ministry are not living right, I disuade them from participation in service, that would consist of them serving. Day Care, Choir, Instruments, etc. They also have to attend and participate in Teen services.
To speak on the Evidence being speaking in tongues, did you know there are tribes in Africa, and South America, that are not Christian yet speak in tongues during Christian activities. I said this to say... Tongues can be imitated.
Now, I wholeheartedly believe in "Speaking in Tongues" is a gift of the Spirit that requires, interpretation like jeremyr said.. The phenom is God speaking through a vessel in a unknown tongue, that must be interpreted so the body of Christ can be edified.
Their are also two other forms of tongues according to the Bible. There is "Glossa" which is God giving a person the ability to speak another language that they had not prior spoken. Ofcourse in this instance there is no interpretation necessary. That's like if I got off a plane in Russia, and began to preach fluent Russian.
Then there is what is called "Praying in Tongues" Paul, says in this instance the believer is speaking mysteries to God. It is the believers spirit, praying to God, and edifying the believer. The believer is making utterances, even they don't understand.