.... until their language level is ready!
There is no-one who likes children to say 'please and thank you' more than me. My own children are very polite and know that they are unlikely to get what they want without these important social niceties BUT I do not advocate parents of children with delayed language or early language to expect their child to use them until much later. 'Please' and 'thank you' are NOT communicatively functional. It will not help a child to express himself if he is struggling to use single words or put 2 together, to make him say 'please' and 'thank you'.
If a child is saying single words it means that they are at an early stage of language acquisition. If, therefore, you ask them to say 'Please' or 'Ta' before you hand over the required toy, piece of food, drink or whatever, they will be very confused. Much better to name the item or say something related and relevant to the moment, so they can learn the appropriate vocabulary.
If I am trying to encourage a child to put 2 words together such as 'more + biscuit', they won't be able to say 'more+biscuit+please' until much later because this is actually 3 words together. I am not aiming for 'biscuit please' because that is not as functional. It's much better to encourage them to say 'more biscuit'. This shows also the child the idea of combining words. 'More' is an excellent pivot word because you can have more of anything e.g. 'more cuddles, 'more ticking', 'more jumping', 'more juice', 'more bubbles'.... anything.
I don't even teach the signs for 'please' and 'thank you' until a much later, more sophisticated level. I have worked with too many children with special needs in the past who constantly tap their hand to their mouth to ask for something.. anything.. with a generic sign for 'please'. Much, much better to teach the vocabulary (signed or spoken) for the item they might want.
Giving the good example is very important and is sufficient at this stage of development. I would welcome your comments though!