God would never send children to hell. However, this does not mean children will stay in heaven indefinitely. If God put the devil with one person, He will put the devil with all. It is just a matter of 'when'.
Scripture is pretty clear on this.
1. Jesus says in Matt 19:14 that heaven belongs to the children.
2. Moses says in Num 14:29 that God spared all under twenty in the wilderness.
3. Peter says in Acts 10:34 that God is impartial.
4. Jesus says in Matt 24:31 that after Armageddon, at the end of the tribulation, He will gather all on earth.
5. John says in Rev 20:7 that satan will be loosed on the earth at the end of the Millenium to test mankind for the last time.
When you put all this together, you realize the following:
The millennium is not a period for the wicked to redeem themselves in and it is certainly not a period for the righteous needing to further prove themselves. It should therefore be obvious that God use this period to allow all children under twenty, to grow up to maturity and toward the end of it, be tested by the devil.
As God is both impartial Acts 10:34 and righteous in all His ways Psalm 145:17, it should be unthinkable that He ever send a child under twenty to any sort of hell.