I have the same thing happening at my house. My daughter's significant other is bringing his two children for the Christmas holidays. 5 & 7 I'm not as ambitious as you, I'm making them one I Spy quilt. This dovetails with the ISpy book my daughter is getting one of them. It helped that rbweise was having an ISpy swap and 80 different 5" squares are showing up at my house any day now.
Ask, Ask what they would like. Think twice about using blue and pink. Many kids aren't thrilled with that.