Parents bring up their children and try to give them a decent career with the available resources at their command feeling it as part of their duty. In this case are the children indebted to parents and have to repay it in another life?

Every human being has four debts. One of the foremost among them is one towards his parents. So taking care of the parents will not become a karma that will be carried on to the next birth.

Is it enough to serve and pray to our own parents and invoke their blessings and wishes instead of visiting the temple and pray to God?

You should serve them assuming them to be verily the form of Sri Krishna and Radha. You should strongly believe in the same and should not even have an iota of doubt about it. If this is the case, then it is enough to serve and pray to your parents alone.

Why is our mother considered as God?

Not only your mother; your father and Acharya are also considered God. Since you have come from her, she is your creator. So she is the visible God to you.

On the occasion of Guru Poornima, kindly advise us on how parents can impart good moral values to their children.

In days yonder, joint family system was in vogue. The elders in the house imparted moral values to the younger generation that lived with them and made them spiritually sound. The very family surroundings and the elders provided a good learning environment for the children.
In today’s fast-paced life, the joint-family system is no more prevalent. In such a life-style, there are few opportunities to impart moral values to children. But, whatever is implanted in the hearts of the little ones remain with them until the end. It is the values that are taught to children that keep them on the right path. Even if the surroundings and situations may distract the youth from his right path, he will eventually come back to the right track, thanks to the values he had imbibed in his childhood.
So it is the responsibility of the parents to inculcate good values in the youngsters right from childhood. They should tell them the stories of great Mahans like Ramakrishna Paramahamsa, Ramana Maharshi, Chaitanya Mahaprabhu and so on. They should include them in all the religious and spiritual activities that they conduct and participate in.
They should always pray to the Guru for their children.