Sonnia's aura is only 3" and is really just there to discourage enemy models from coming in and locking her in melee. I generally focus on keeping my other models out of it. The Witchling exemption means you can park a Thrall next to Sonnia and keep her safe, if you need to. Stoke the Flames lets you funnel healing and focus into a Thrall which is great. Realistically Sonnia's bonus actions won't be affecting your Witchlings past turn one, Sonnia has a 4" move and wants to be at the back blasting things, the Witchlings are all melee models who want to be in the thick of it. They're rarely in the same part of the board, in my experience. And, sadly, the Witchling Stalkers are somewhat slow, somewhat fragile models that want to be in melee, which isn't a great combo unless you manage to summon them. They're great models, but they tend to die before they can get much done.
You're also probably ok to drop down to one Handler, they're good models but as Witclhing support pieces you're better off taking two Thralls with one Handler to support them. A second only adds so much without the Thrall to buff.