What are the effects of ritalin (methylphenidate) on a 6 year old with add and when does the medication start working?

Immediately : Stimulants act by stimulating the cortex in the brain, in add this is like stimulating the brakes in a system. They usually start working short after start. They block the appetite, increase pulse rate and blood pressure and may disrupt sleep , I am not aware of studies that followed people up to adulthood, so we don't how they affect the brain as it develops from childhood to adulthood.

Immediately: Ritalin or methylphenidate is an immediate acting stimulant medication and you should see an effect within hours if not days depending on dosage. You should expect change in attention, concentration and distractibility as well as impulsivity, hyperactivity and forgetfulness. Side effects; decrease appetite, change in sleep and rarely neurological tics or twitches.