Your dwarves have too much to do. Kill one of the projects causing so many hauling jobs and wait til the other finishes.
Get dwarf therapist if you don't have it already. I split my dwarves into 3 groups. Haulers, specialists(farmer, crafstdwarves, etc), and military(up to 5 full squads). In practice i have about 50% haulers and the rest labourers and military. Things get moved relatively quickly since EVERYONE will haul and if it becomes a problem, i just switch off a specific dwarfs hauling labours and he gets to his real job.
A goblinite stockpile next to the workshop is a must but i'm always tinkering with the settings to keep things in or out depending on all my other stockpiles. On the other hand, an atom smasher is a good solution too. There will always be more goblinite for you to melt. You don't have to stockpile it either, just stockpile what your dwarves need and let the rest rot in the sun until your dwarves are free to haul all the copper items down near the workshop, then the iron, then the bronze, etc... The idea is to limit the number of things they have to do. The more there is the slower it all goes.
As for children... My fortress grew to 350+ and i have a number of children running about as well. Pathing alone will drag me down to 10 fps if everyone is hauling for some big project. Thankfully with 350, the lag dosen't last long. I don't have the heart to squash the kids but i did set [BABY_CHILD_CAP:5:1000]. That's a maximum of 5 children at any time ever. The rest will just have to die of old age before new ones will show up.
Yes you can still get migrants but once you hit a population of 200 (actually, i got migrants at 210 but it stopped right after) it will stop but new births will carry you well over that.