You are finally going to build your dream home. Don't just call a builder. There is work to be done before you make that call.
Building your dream home is a big step. It is a huge investment that should give you dividends for a long time. It should have the amenities you want and MUST make you happy. You worked hard to be able to afford it and you want it YOUR way.
The first step is deciding what features you want in your new home. Include your partner and children in the decision making process. They are going to be sharing in this experience and living with you. This home belongs to everyone. Write down everything you want in this new home. Don't forget the outside and the landscaping features as well.
The next step is deciding what features are absolutely necessary. What features MUST be included in order to make the house livable under current circumstances. But also consider the near future, especially if the family may be expanding: ie: another child, a parent coming to live with the family. Perhaps now is the time to include that 4th bedroom, and extra