Comes with answer but no full solution. However, I think it is good enough if you are looking for free resources.
As for the level of difficulty of each question, it really have to depend on your child's math ability. Brain Teasers involve logical thinking and deduction. Having said that, some questions do involve some pre-knowledge on Math concepts as well.
Enjoy and you can decide whether which question to let your child try. He/ She may surprise you, who knows?