Harbor Freight has a few different options for solar power. Cheap Chinese stuff but it certainly isn't gonna break the bank. I have one that folds up like briefcase. I have a Rayovac battery charger I picked up at Walmart for cheap then all you need is a 12v to 110 conveter. 18 Volt to 24 Volt Solar Charger
I have both that I got from Real Goods years ago. The NiCads eventually wear out and are a disposal problem, being toxic and all. NiMH has better power density and will likely last longer. It's handy parking the charger on a windowsill and always having freshly-charged cells ready.
You might want to mark rechargeables some obvious way (we use a green duct tape stripe) so the kids don't accidentally throw them away.