Does this design reflect a(n)...
• well developed and easily understood concept?
Your conceptual solution clearly communicates the massage that you intend to convey.
Please make sure that the image that you have used is in fact a landmine. From my past experience (I spent 1 ½ years in the military), the image that you used looks like it is either a grenade or a mortar shell rather than a landmine. The shapes of the different devices differ radically. Grenades can be set up as landmines but it is less common.
Also look at your use of language in the headline. ‘Peek-a-boo’ is a common vernacular phrase in North American but is not necessarily universal. For instance, in my region we use the term ‘hide and seek’ and ‘boo!’.
• level of social engagement?
Good. See above comments to make it even more powerful.
• understanding of the issues reflected in the poster?
• sense of internationalism and accessibility?
Good. You might want to consider a series of posters that look the same but changing the headline to contextualise it to regional vernacular language.