Methylphenidates and amphetamines are two different types of stimulants — they are much more different than just the brand. Almost everyone does well on one type or the other, but not both.
The best practices when prescribing stimulants is to start with the lowest dose and increase only if and when needed. The lowest dose of Adderall XR is 10 mg, so that is what your children should have been prescribed.
You definitely need to find a new clinician to work with your children on ADHD medications.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism