Thank you for the additional information.
Your first step is to talk with your neighbors. You may have already tried that, but if not, approach them in a friendly but direct manner. Let them know that you wish to work out a better arrangement. Then spell out what you want. For example, if you want to use the wash line two times a week, ask them which days they would like to use it then let them know you will be using it the other days- the full line, not half. For the lawn, can you get a small fence for your side? They do sell half fences or lawn fences you may be able to set up to mark your side. Then if they cross it, you know it is deliberate.
If you can, try to set up your own laundry line, watering system and other personal property that they cannot use. Set up a sign in your yard that says no trespassing. Try to make your boundaries clear so the kids know where they are. Kids sometimes don't realize that any yard they see is not theirs.
If they are not willing to work out a compromise, then you may need to start keeping track of what they are doing. Write down each offense with the date and if you can, take pictures. Date each picture. Then write a letter to them stating very factually what you see, state the evidence, and suggest they work with you.
If that fails, you may need to file a complaint with the homeowner's association if you have one. Or you can complain to the local county or city offices. Make sure you bring evidence and your written notes. Although they are not doing anything to be considered harassment, you can get some advice from the county or city as to what your rights are.
Worse case scenario is hiring a lawyer or moving out. Although you probably don't want to do either, especially since they are only there part of the year, it is an option to keep in mind. Sometimes a letter from an attorney with a well worded warning is enough to keep others away.
If this situation has made you upset to the point of having trouble relaxing, consider talking to a therapist, even for a brief time. You can talk out your stress and find ways to relax. Trouble with neighbors can often make us feel nervous and upset. Having your peace disturbed at home leaves you feeling that you have no safe place to go. Talking to someone can help you feel you have someone on your side and help you learn to cope with the stress.
I hope this has helped,