How do I ensure my wishes for my minor children in a Will are carried out and not given to the surviving parent?

Your goal can be accomplished by including some kind of trust inside your will.  This is called a testamentary trust.  You could also create a separate trust.  If you retain control of the separate trust, can change it, and can terminate it, it is called a revocable living trust or inter vivos trust.  If you create a revocable living trust, you can write the will to "pour over" everything from your estate into the trust.  You can write the trust terms in many different ways to accomplish what you desire.

I read your question as a concern that the surviving parent not get control of any assets you leave to your children.  Without consulting with you about your specific circumstances, I can only make general comments.  My first concern would be to make sure that the representative you name in your will to administer your estate is not that surviving parent and that this representative or another independent person become the trustee of any trust you create for your children.

A couple other cautions -- First, if you need to create a trust for tax purposes, you need to have a lawyer.  Do not attempt to do this yourself with any kind of form.  Second, I generally advise against giving substantial assets outright to children at 12:01 AM on their 18th birthday.  In my experience, this does not work out very well.  I generally prefer to give the trustee discretion to hold assets or disburse them for needs or education rather than hand an 18 year old a bunch of cash.

I hope this helps and good luck.