Firstly, i bet the kids won't get a lookin on it!!! you will keep it all to your self!!
secondly, do not v@rn*sh it when clean, especially NOT on kiddies equipment, as it flexi's it will just crack and craze, oiling it will be much better......
as for cleaning it, you could try a wire brush, BRASS, not steel!!!, and is it MOULD or just Green algee!!!
if just green algee then you could try using a bleach/water mix, and a hard bristle scrubbing brush!!!!!
is it a round or square section timber!!!