Ack! You sold your 50mm prime? You don't like it for children's portraits?! I think my head may explode lol. That's probably my most go-to lens when I am shooting kids, and after that the AI-S 50mm 1.4 is next for shooting older children/teenagers who can hold still enough for me to MF.
Can I ask why you find it difficult to shoot with? I'm just curious and kinda baffled lol. Is it really all because of zooming with your feet? If so, that's truly a shame because primes are always so flattering for portraits (and much sharper at large apertures, IME).
If you're going to drop serious money on a lens, why not the 24-70mm 2.8? It's the next lens on my Tax Refund list, probably one of the few "portrait zooms" I'll ever purchase.