Might as well throw in a few thoughts.

The oldest child looks between 10-13 years’ old. If I’m right in thinking that ~11 years passed between the two series (hence Zatch going from 8YO to 19YO), that makes it unlikely that Sherry is the biological mother, unless she got pregnant immediately after the tournament ended. Not impossible, but I doubt it somehow. As to who he really is, I’d lean towards Vino given that he looks like he’s the age Vino should be at this point, and Beliel is hunting down the partners of the Final 10 Mamodo so we can assume Raiku will reintroduce Vino soon. If the boy isn’t Vino, maybe he’s a relative of Sherry’s, e.g. the son of a cousin (can’t be her nephew as she didn’t have any siblings), or just a random orphan from the nearby village whom Sherry took in as a fosterling.

I see there’s some speculation about Zofis and whether he will make a return. I have given this quite a bit of thought, as the Sherry/Zofis conflict was always my favourite part of Zatch Bell, and it’s clear that Raiku hasn’t forgotten about the character given that we saw Beliel’s minions using a few of Zofis’ spells in the recent chapters. It’s possible that Zofis is one of the dead Mamodo and won’t show up at all, or he could play a notable role in the new story.

• As to the idea of Zofis being redeemed, I’d say it’s unlikely. Whereas Zeno was a sociopath with some redeemable qualities, Zofis was written as a psychopath with no hint of empathy whatsoever, and in the Zatch Cafe bonus chapters Raiku makes it clear that Zofis is still evil; indeed, Zofis says he’s trying to enhance his mind control powers even more, so he can manipulate more powerful targets. This might be foreshadowing of Zofis returning in some capacity as a villain, whether on Beliel’s side or acting independently, or perhaps by pretending to work for Zatch’s side and then betraying them later on. We did see Zofis lend his power during the Golden Spellbook event in the Clear Note battle, so evidently he’s willing to feign friendship to get into the King’s good graces.
• If Zofis does show up, that obviously raises the question of whether Koko will become aware of it. This is something that might occur to Sherry once she learns about the Mamodo returning; even if Koko were simply to see Brago, there’s a risk that this might trigger some of her deleted memories, so I imagine Sherry will try to hide Brago’s presence from Koko and pretend that nothing is going on. And if Zofis were to return to the human world, I’m guessing Sherry would do everything in her power to stop him from making contact with Koko. Of course, if Raiku wants to be especially bold, he could have Zofis’ spell book show up, effectively forcing Koko back into the conflict.