Advising students taking part in a competition: how to provide direction, but not to contribute?
I have only worked as a TA to help students on an individual exercise basis, but I think the idea is kind of similar to this. In the case of a competition like this I can only speak from my role as being supervised. 
However, I would say that the most important thing is to ask questions, such as "Why do you think that step X that you propose here would be a good step?" or "What results do you expect by doing this thing Y you're doing?". This should make them think about what they are doing in this competition and why they are doing it.

Regarding the presentation, you can of course always give them advice about how to present, how to style their slides, and how to convey the information they are presenting in an efficient way.

All in all, with most things you help them with, just ask yourself if you're telling them how to do it. If not, you're probably fine I guess.