there are many children who have difficulty with handwriting, but keep in mind that there are many adults who do not write neatly as well! Not all children with ADHD have poor penmanship or fine motor control issues. What usually happens is that these children find writing to be a tedious task, and they tend to rush to finish. Thus the problems with handwriting are often a symptom of their impulsivity. Consider how long it takes for an elementary school child to produce a sentence, paragraph or report, and you can see how written language is a particular challenge for children who have difficulty sustaining attention.
If a person can write legibly, then elementary school is the only place where adhering to specific penmanship standards matters. I routinely recommend that parents of children with ADHD get their children touch typing as soon as possible. This is an excellent intervention for these children. Touch typing allows children to produce a higher quality product in a far shorter period of time, which is an important factor in helping a child with ADHD. What matters more than the penmanship is that children with ADHD master how to organize their writing and write for a specific purpose. This is usually the area of significant difficulty for kids with ADHD.
Your children are not likely to qualify for OT services unless they have fine motor issues that have a specific, measurable educational impact. Determining educational impact is the factor that determines if your children are eligible for a related service (such as OT) under special education law. You may wish to request an OT assessment if you see that your children have significant difficulties with fine motor tasks (such as immature pencil grip, if their hands are quick to fatigue, or they have trouble with visual motor integration) that actually do impact their ability to make progress towards mastering the curricular objectives.
Check out Dr. Larry Silver's Advice to Parents on ADHD as well as the website ldonline.org to learn more about ADHD.
Gross and fine motor skills are issues that some ADHD children have. You need to get an Occupational Therapy (OT) professional to evaluate your children. The OT specialist can provide therapy to help your children with their hand writting. The children will see the specialist on a regular basis (once or twice per week). They will work on the hand writting and other fine motor skill issues. It can be expensive since it is private OT. Sometimes, health insurance will pay for this type of OT therapy. Also in US, you can take the private OT evaluation to your school and request OT help through the school system. It is your right to make sure that the school is doing everything to help your children. The school should NEVER give up on your child. The law states that "no child will be left behind" and the school district will need to provide OT services if the children need them. Good luck and try both paths (private OT and school OT).