Well, despite whatever the recruiter (or anyone else tells you) you should know - as a prospective military officer - any form of deception, omission, or dishonesty is WRONG! And, to answer your question - intentionally omitting or providing inaccurate information on your DODMERB can cause you to be permanently disqualified for military service. Now, in practicality would the military find out about an ADHD omission - maybe not but do you really want to risk it? And, with the forms you are allowing the military to gain access to your past medical records. Maybe they request pediatric records about something completely different and the ADHD is referenced on a page in your files…. If it does come out that you purposefully omitted the condition to better your chances it will not go well.
You are not the first candidate to ask a similar question on these forums. I always read the question and shake my head - because you knew before you asked that this was the wrong thing to do before you did it. Now, you are concerned about being discovered. Your condition may never be discovered and you may make it into and through ROTC and become a military officer some day. I do not believe starting your path towards becoming a military officer is best represented by your lying on forms to get past the medical. If you do become a military officer, please do not take wrong or inappropriate actions because someone else tells you to! You and I both know that the right step to take us to say you have remembered additional medical information and attempt to amend your file. It may or may not effect the end result but will give you a clear conscience for years to come.