craftgrrl | Advice?

I need to make some little sculptures, about 17cm high, casting them with a latex mould. They will be painted (by kids) and then left outdoors, so need to be waterproof. Does anyone know:

1.) What I should cast them in? I’m looking for something like Plaster of Paris, i.e. white, and giving a surface on which I can paint, but waterproof. I saw something called Cassini’s Outdoor – is that any good?

2.) What they should then be painted with? Poster paints? Acrylics? It’s OK if they then have to be painted over with some sort of varnish / sealant to make them waterproof, but it’d be great if there was a paint that’d be waterproof straight away. And it’s be absolutely perfect if it was very safe for children, i.e. not toxic.

3.) If they do then have to be painted with some other sealant to be waterproof – what’s best?

I’m looking for materials that are available in the UK – and that will stand up to the UK climate! Thanks.