What a load of politically correct bollocks on this thread.
So you'd all be happy for a violent man who had seriously harmed his child just a week ago to be at your local toddler group so long as he had MH health issues to excuse his violence? Get real.
As for "the mother wouldn't be in a relationship with him if it had been serious," so all those women who stay in violent relationships and for who it takes dozens of violent incidents to leave, is that different? No didn't think so. In fact I'd imagine that the diagnosis of MH issues makes it even harder to leave a violent partner because of the guilt over leaving someone who is ill. Guilt which is being suggested on this very thread.
The facts as the OP knows them are that a week ago this man seriously harmed his child, was sectioned for a period, and is now back with his partner who has seemingly allowed him back into her life (I wouldn't.) If she'd taken him back after he'd beat her up would people have the same attitude?
Of course people with MH issues shouldn't be excluded from society. But people whose MH issues mean that they are a danger to others do need boundaries, and I certainly wouldn't be assuming that everyone had worked together to ensure this man was safe, or even that they would have been aware he was even going to the toddler group.
TBH OP if you know the name of the mum I would give SS a call and express your concerns. It's entirely possible that they are unaware that this man was even at a toddler group so soon after harming his own child. While there may be stipulations, nobody is there overseeing them 24/7 so it's entirely possible he may still be considered a risk but that he wanted to prove himself worthy of going to a toddler group esp if he knew his partner has been telling all about what he's done.