I recall a cute introduction to the work of Miryam Mirzakhani that is written at the level of a younger student. Something like this could be adapted and incorporated into a 11-year-old's math curriculum. Here is the link to the infographic on Matific. It talks about surfaces and the genus of surfaces and is reasonably well explained. Keep in mind your 11-year-olds might not have a firm grasp on the difference between two dimensions and three dimensions. Upon arriving there you will need to evaluate what the students are able to understand by possibly asking a few fun get-to-know-you questions that double as assessing their geometric ability. They might not remember the difference between a circle and a sphere without being reminded.
I remember that when I was 11 or so, my dad had me imagine games being played on different surfaces and how the rules would change. He also taught me about donuts and coffee cups being topologically equivalent, and had an amusing obsession with the Klein bottle and Mobius strips (I think this has some decent explanations for that level, and some of the examples it describes could be used as manipulatives.)
Additionally, there are many manipulative topological puzzles that 11-year-olds could spend a class period on trying to solve in groups. You can get these (with metal rings, ropes etc) at Barnes and Noble or many novelty stores if you don't want to order online. In fact, even the Human Knot might be a decent introduction into knot theory. If you did it multiple times or multiple small groups, your students might realize that not all of their "knots" could be fully untangled into a single circle. Students could also draw Celtic knots or do the Handcuffs puzzle with each other.
Regarding triangulation. Small groups of students could have a balloon and a sharpie. Instruct them to draw on about 4-10 dots then connect as many dots as possible to create a triangulation, then count dots and edges. You WILL have to give extremely clear instructions and demonstrate this along with them. You may also want to give them two colors of sharpie or give them a tool to help them avoiding miscounting so the entire time isn't spent counting edges. Doing in groups will help keep them from getting over taxed -- each student could count something different.
Apologies that I don't have many examples of the advanced concepts which you have mentioned, but hopefully these are some hands-on ideas that can get them excited and interested. In general, because these 11-year-olds are struggling in math, manipulatives and hands-on activities are a good idea and will engage them much more than doing examples. One hour isn't much time with 11-year-olds, and unless you're planning on giving a lecture (which wouldn't be age-appropriate), I'd plan on covering maybe one or two of those concepts.