Should children be thankful to their parents for what they do for them

It's very important to understand that a child is a new soul that comes into this World through us, but not for us.
With so many different ways available to avoid getting pregnant, why do parents want to give birth to children? Most of the times, parents reproduce to continue their lineage, or to have someone with them when they are old, or to have a successor to their wealth, or to bring some change to their lives. From which we can inference that it is for the parent's own motives that a child is born. So, if the parents bring a child into this World, it is their responsibility to take care of the child. How well they do it would define the future of the child, which in turn would define theirs. 
In my opinion, every parent must try to instill the virtue of gratitude into their child, so that they could be grateful for whatever they have around them. But, they must not be the ones to go and tell their children to be grateful to them because they did so much for them or because they brought them into this World. Moreover, as much as the World is beautiful, it could turn out to be a very harsh place for some. It was solely the parent's decision to make, and not the child's. So if a child is grateful for what the parents do for them, the parents must be grateful to them the other way round too. 
A child is a new being who comes into this World with the help of parents, but that does not give parents the authority to rule this new life or to expect anything out of it. Parents do this investment into their kids and want it to bear fruit when they need it. But, that can go the other way at times, when the child wants to part ways and live his life on his own terms. While some parents are prepared to handle this situation, it could be very saddening for few.
I am a mother of two, and whatever I do for them, I do it out of love and responsibility, and not with the expectation that my kids must be grateful to me. Having said that, I still have an expectation out of them to love me and take care of me when they grow up. Whether they are grateful for what sacrifices I made in my life for them, or not is up to them. I would be very happy to receive that appreciation and pat on my back for being there for my kids. But that does not make me ask my kids to do it forcibly. I do not go to them and ask them to be grateful to me under any circumstances. I do not tell them that I did so much for them that they must be thankful. Thankfulness is the virtue that I teach my children instead, which might let them into appreciating us when they grow up, if we are worthy of it. 
Parents today are equally aware that the children would be leaving the house and moving on in their lives, and might not even be there to take care of them in their old age. Which is also very understandable from the children's perspective.

Like they say, let the love fly and if it comes back to you, it is true love.
Children will always love and cherish their parents as long as they are let free and not held back by the parents for their own motives. Love them, cherish them and let them be free! Let them decide what they want to be to you! Love will automatically come back to you.