It will vary depending on your state, but it should not require a trip to an attorney's office. Legal Zoom or similar (haven't used Legal Zoom myself) should be able to get you hooked up for your state.
Regarding the kids thing, you should be able to write it so it is per stirpes or per capita ( https://en.wikipedia.org/wiki/Per_stirpes ). Unless one of your kids or grandkids ticks you off, you shouldn't have to revisit it in the future.
OP, if it were me I wouldn't take legal advice from someone relying on wikipedia for a legal definition. I highly recommend that you find an estate planning attorney that is licensed to practice law in your state. He or she should be able to give you a quote for the documents you need and advise you regarding the laws of your state. Also, things change in the law and your local attorney should be able to identify it faster than LegalZoom can change their form documents. If you use their form document and make a change somewhere, it's very possible you could inadvertently change the meaning of another provision in that document, causing two separate provisions to conflict. In other words, a legal shit show. I'm all for DIYing tons of stuff, but please think through using some form you find on the internet. And yes, your will can contemplate the existence of future children so you don't have to redo it when subsequent children are born.
When you choose an attorney, you should be provided with an Engagement Letter, which will describe the terms of your agreement, including cost (per hour or flat fee) and what documents and/or advice are included in the costs quoted. You should get this engagement letter (at no cost to you), signed and in effect, before beginning your relationship with the attorney. This Engagement Letter is what will specify, in detail, what you are paying for and in what amounts/on what basis.
I'm a tax attorney and I had an estate planning attorney draft my will and other related documents. This is a specialized area of law and the attorney you work with should be able to structure your affairs in the most tax-efficient way possible to reach the outcome you prefer.
If you don't know any attorneys to ask for a referral in your area, contact your city or state's Bar Association and they should be able to refer you to someone.