How sweet of you to think of this. I can think of so many things to add to your list, but I don't think that's what you're looking for. The 4 yr old may be harder to try this, but the 6 yr old is perfect. Ask them what they'll miss most in their rooms when they go stay at Daddy's place. Do they have a favorite doll,car,book, blanket...etc. that don't come to Dad's when they do? Maybe your brother can help out with the 4 yr olds favorites since he/she is a bit young. If they are sharing a room at Dad's place, maybe an inexpensive table and chairs for them to play on. Make a sign with their names on it that reads.." _______'s room." Something that gives them claim to this new house. Making it personal. Get Dad to ask the 6 yr old advice on where to place some of the not so important furniture for example. He can get his younger child to help set up their room. Also, a simple pet would make Dad's place special. Something like a goldfish or hermit crab. Simple, yet special.
Hope I've been of some help for you.