I've recently started a photography business, not because I can take pictures with my phone and instagram, a legitimate business. How will the tax man feel about me employing and paying my small children as models to boost my portfolio?
As long as they are just doing hours here and there I don't think labor laws are really going to come in to play. There are obviously child actors/models.
It seems perfectly reasonable to pay them something 'reasonable' for being a model for your marketing photos. As long as you are paying taxes on your earnings and the taxes are filed appropriately for your children it should be fine.
But, really ... how much modeling will they be doing? Just up their allowance ;)