First, you will have to check with your state regaurding child support for your 18 year old, as some states very on the age and being in the service, etc..
Now as far as the other child coming to live with you, be very careful with the noterized letters, as some places will accept them and some wont. If you are both in agreement to the child living with you, it is a very simple thing to take of in court (your child will not have to go to court). You would go into your local Family Court and get paperwork to modify your current custody papers. Another reason for you to do this in court is that the child support will change when the child moves in with you and the courts will record that. It is not a big deal when both parties agree to it, you get your court date and you both go into court and the judge will ask you both if you agree with the change, etc.. and then it is done. No one will need to testify or anything, as long as there is no problems. You could have your ex sign the papers and you both can use them as a guideline to tell the judge what it is you are requesting. It is all a matter of getting everything in black and white so nothing comes back to bite you later down the road. Good Luck to you!

