It all depends upon what 'justified' means. Would you rather your little sister to suffer and die, or many kittens? I'd pick the kittens. I value my little sister more than kittens. There is no right or wrong here, there is only opinion. Also, the basis for that opinion is not entirely rational. Questions like this are important to ask, but they will not reveal some greater truth, or even be a reliable guide for the future. Still, it's important that they are asked frequently, because our answers may change over time. Of course, I've written on this topic.
Right. The fact of the matter is that most research is done in vain, leading nowhere, but destroying many animals in the process. We can quibble about whether that's salient or not, but it must be considered in the calculus. There's pretty much no disease research that proceeds by the process presented in the question above. Even though it's ostensibly for kids, I think it's giving children a wrong impression to assume that there are only two outcomes to be had. The world has many shades of gray, and I think even children can appreciate some of them if given the opportunity.