I have a 2 year old. I would say that be prepared to take your time, try to avoid making the older child pose. You'll have to just pick your moment. If you can get some photos of the baby asleep, focusing on the details - facial features, feet, hands, are good. Lots of "soft" like a soft plain blanket, that sort of thing. They need to look cosy. I keep my 2 year old (nearly 3) amused by giving her an old compact camera that still works. She loves taking photos with that!! You will have to let the 5 year old just do his/ her thing. I would probably get the baby shots started and wait for the 5 year old to come and be nosy and start interacting. They usually do!! Then you can perhaps ask the 5 year old to "help" with the baby shots - you know - could you just tuck the blanket in a bit - that sort of thing - and sneak some shots of both of them looking natural that way. The baby will probably wake up, so you can get some of them both awake together. Then if you can get the 5 year old outside to just play whilst you "talk to mum and/or dad" (i.e., take photos without the 5 year old noticing too much), you'll probably find you get plenty of chances for candid shots. Clothes are quite important too - make sure the 5 year old is wearing something that looks good (maybe work this out with the parents before the day of the shoot) and is comfortable. Sometimes things like a little stool can be a good "prop" as the 5 year old is bound to want to sit on it....
Main thing is don't try to do a "formal" posed shot as probably won't work. If you want a whole family shot maybe ask mum or dad to give 5 year old a piggyback!!