Firstly, unskilled doesn't mean useless. There's always work to be done, and some dorf with poor skills nevertheless have great potential. In addition to that, they learn fairly quickly (a lot faster than real life humans...).
If you're afraid of running out of food and booze, increase your agricultural production as quickly as you can. Tundras have somewhat poor selections of boozable (and edible) plants, but you should still be able to collect some and build a surface plant farm (I build mine by channeling away one Z level over an area, build a roof over it, and use it as my courtyard/surface farming area/pasture).
By gathering surface plants and process them by a still you get seeds for your (surface) farm plots as well as booze (and variety) for your dorfs, and a savage surface isn't particularly bad (it can be if it's evil, but any evil can be scary).
According to my understanding of how Toady has implemented aquifers, there should never be both heavy and light aquifers directly under each other, as you'd either get light, heavy, or none (you can get the other kinds in a different part of the embark if it's "ruled" by a different world tile). It's possible to hack both light and heavy aquifers above each other, but I assume you haven't done that. Also, it's rare for DF to have multiple aquifers, although you can get one in sand/soil at level 2, have non aquifer clay at level 3, and then sandstone or conglomerate stone (there might be a small number of others) below that, though, but I don't think I've seen a naturally appearing rock aquifer underneath a non aquifer rock layer (although it can be hacked).
Just let the kids grow up. Create a burrow for them to keep them inside the fortress until they mature, and release them from the burrow at that time.