I would say we cannot give a yes, or no answer to this question, as there are many issues to take in mind.
Yes, from a Fiqhi perspective, there is no problem in itself with women watching men "playing sports." There would be a problem if the men were topless, for example. There is also no shar'i requirement that every woman who attends, Muslim or not, must observe Hijab. Unless of course it is an Islamic country where Islamic dresscode is applied to all in public.
An issue that we need to look into is whether the atmosphere is appropriate for young boys, or girls, or even women. Will there be excessive swearing and offensive language thrown around. Is the mixing between genders so much that it would be an inappropriate environment for a mu'min/mu'minah to be in.
Is it culturally accepted, for women to go to such a tournament, or will it be taken as something bad, or she will be criticised, or spoken ill about.
It is clear that we are influenced and dragged into today's sports culture, and we might have an interest in following some sports group, or be serious fans. At the same time, we have religious and moral principles that we must never abandon.
This applies to both genders as well. Our level of modesty, evaluating where we should go and where we should not go, looking into the shar'i, social and cultural aspects of what we participate in, and other important factors.
More can be said about this, but in general the mukallaf must try to look at this from different angles, and like everything else be as cautious as possible, choosing what would bring him/her closer to Allah ta'ala.
With prayers for your success.