Such a wonderful prototype with lots of potential! I would suggest playing a bit more with the ergonomics of the swings, especially the curved edges and how exactly they are to respond to the human body. Perhaps, it would be better to have multiple swings at different scales, which would fit both small children and bigger users. This, i believe would ensure that bigger, stronger folks do not monopolize use of this popular feature. and discourage younger kids from interacting with the swings. Also, i would suggest adding a rubber-like surface to better cushion users as the use the swing.
In terms of materials, I very much appreciate the use of local and re-purposed materials. Have you though of using bamboo, which is indigenous to Ghana and has a fast-growing capacity. It also has great structural strength, comparable to mild steel and concrete, in addition to having a "bendable quality." A variety of treatments can be used to preserve the material from adverse environmental conditions. The safest and "greenest," especially since it would require such close contact to human skin is a "lime wash, which fuses water and quick lime. This treatment offers an array of benefits, from discouraging termite presence, is anti-fungal, insect repellent, fire-retardant, adapts to weather changes and good thermal/sound insulation.