Go ahead and approach the parties and explain your plan, be sincere but make it clear you are willing to accomodate whatever they stipulate to. Dont forget to offer portraits of the kids to the mother(gratis, of course). You could offer to photo the kids seperatly and give them their own page in the album. It wouldn't hurt to offer copies of all the family to them as well, as it is important for the children to know who their family is, and be able to put faces with names.