Is it your DPS simply can't keep up because tanks and healers may not be helping like they did in the first transition? You can't ignore the Scions but you don't particularly need to do much with them other than pick them up. Since the Scion tanks themselves can't be targeted by the fire debuff they are still free to DPS/Stun the Sons while tanking the Scions. In fact, the tanks should definately still be helping with the Sons in this transition, they just have the additional responsibility of picking up an add that is tanked and maintaining sufficient threat on that add. The killing priority is still on the Sons. Both Scions should end up on one tank so that the other is free to pick up Ragnaros when the transition is over. Ideally you would like to have one of the sons slowed significantly (below 50% and left alive) so you can widdle down or even kill one of the Scions before entering phase three however it isn't absolutely critical. Once phase three starts you clean up the Scions so that tank is freed up to taunt the boss before the active tank has too many stacks of the debuff.
You don't ignore the Scions... you tank them.
You don't ignore the Sons... you kill/stun/knockback/deathgrip them.
Having threat on one mob while killing another shouldn't be an issue for a tank at this point. If you're tanks can't handle holding threat on one mob while helping with others I think that would be a larger concern.
While more of the burden in the second transition is shifted to the DPS because the tanks have additional responsibilities and the healers have more damage to heal, this does not mean that healers and tanks can't help at all.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.