To be honest you are going to need a heart of steel to look after a puppy for 6 months and then give it back. People do it (foster carers for charities or people who bring up assistance puppies) but be really careful if you have no experience of this. Especially with your young DD who may not find it that easy to understand why the puppy is going back.
Also keep in mind that you will have to do all the hard work, e.g. toilet training, sleepless nights, socialisation, teaching the pup not to bite, not to jump up, not to pull on the lead, etc., you will need to set time aside for puppy classes, for taking the pup to see the world, etc. and expect some accidents with chewing your beloved furniture.
To me it seems very cheeky of the owners to ask! If they got a puppy they should be around to look after it or they should pay the breeder to keep the pup longer.