Yes, poster paint and then some type of craft spray varnish over the top will work (not sure of the longevity I believe it would last at least a few years). Alternatively you could use something like Dulux Weather Sheild which is water based, can still be a bit of a pain if it gets on their clothes though. The Dulux Exterior Weathershield will bond much better to the metal though.
Would love to see some pictures of them when they are done also!!
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