Do children always work as fast as adults?

[¯\_(ツ)_/¯]

Every time I compared child labour vs. Workers, children do the job just as quickly (i.e. gathering X of wood or coal per hour). This isn't logical but I'm not complaining, so where's the catch apart from a slightly raised Discontent?

Do children fall ill more often or any other negative effect of using them?

[midgkahn]

depending on the job assigned there's a chance of them getting hurt or falling ill.

[Mavrah]

Originally posted by midgkahn:

depending on the job assigned there's a chance of them getting hurt or falling ill.

Yep.
Having children in the workplace means there's risk of workplace accidents. These are scripted events that may cause children to be injured in 'safe' workplaces unlocked by the first child labor law or death with places unlocked by the 'All Jobs' law.
There's also a scripted event where you're asked to give double rations to child laborers, making them even less efficient. Expect even more discontent and lost hope if you refuse.
Just do the child shelter path instead.

[Amanoob105]

As far as I can tell, the main catch to using child labour is you miss out on the bonuses gained from child shelters.
For me, that's one catch too many.

[General Idea]

Child labour gives short term gains but in the long run is detrimental to your peoples state of mind. They want a better future if there is ever to be one. Assigning the children to labour work is a backwards step. Training them as medical or engineering workshop assistants is an investment in their future and the future of your city once the storm has obated.

I am inclined to agree with Amanoob105. Child labour is a bridge too far.

[Khan Boyzitbig of Mercia]

If you go Order then Medical Apprentices is best, if you go Faith the child labour-safe jobs is best.
Engineer apprentices is worst full stop as you will always be able to research everything you need before the end, and in A New Home on normal you can research every tech before the storm hits anyway.

As for the future, if there is no longer any serious threats and there are enough adults then whose to say that the law wouldn't change? It did IRL after all.