I am a little confused about your first paragraph but I will let you know what I think of your questions...
Q: Do children have the same rights as able people?
A: Not sure what you mean by 'able people', adults maybe? But the answer regardless is yes. Every human has the same rights as every other human regardless of age/race/religion/and so on.
Q:How/when do children become able people?
A:Again I am assuming you mean adult, which here is the US is 18. In a stateless society there is no 'legal' status of 'adulthood'.
Q:Does cruelty against animals have any penalty?
A:In your scenario it appears the boy wants to/or has burned his household cat. If it's his cat, I see no 'legal' recourse by anyone to him. Although I would assume/hope his family would punish him for the cruel act. And obviously if it is someones else cat/pet animal, they would clearly have the right to go after said kid for hurting their property.
Q:The risk of property/body damage doesn't allow aggression?
A:I think what your asking is, "Doesn't the chance of the cat coming onto my property while on fire risk me and my properties well being? And therefore I have the right to stop the kid from burning the cat using aggression". (please correct me if I have misunderstood the question.) Unfortunately I don't think you have that right. Risk is daily and everywhere. Is it riskier to drive 60 in a 60 than 50 in a 50? Do you have the right when you're doing 60 in a 60 and someone else is doing 60+, to ram them off the road because there was the potential 'risk' of them hurting you by going over 60? No.
Again, please let me know if I understood your questions, properly, thanks.
I wanted to mean 'capable person' in contrast with incapable person which, these days, includes children, senile and other specific cases. I should have made that more clear.
I think that statement is troublesome because it allows children and other incapable people to make contracts. I can make a contract with a child to exchange an ice cream for 1 once of gold to be payed when he/she reach the age of 20. Or I can trick drunk people to into making really dumb contracts.
That doesn't seem to be an improvement to the current legal system.
I think everyone has a risk threshold meaning it's highly subjective but there's one. Driving 10km/miles faster is not a great risk increase but someone playing with explosives near your kids will make question theses ideas. Not arguing for a blank cheque on the risk issue (like the current system) but there is a limit when you stop caring about rights and just want to stop someone that might hurt you, your family or your property.
AI think this is natural and should be recognised somehow in a stateless society.