This depends so much on the interests of the boy and girl but here's an off beat idea. What about bugs. If you used dragonflies and mayflies and such they are pretty but also can have a sportsman side to them, as in fly fishing. We have a shower curtain with a leafy pattern with butterflies all over it. We all like butterflies and birds and cats and squirrels. Our other bath has dragonfly hooks for the shower curtain. I haven't gone further with it but DH is a fly fisherman and also likes to garden to attract butterflies so it works for us. Another idea is horses since girls often like horses and boys may like the western aspect of horses. Hope that helps, Sunny