Is this normal for trisomy 21. Hand flapping?

Yes. Hand flapping can occur with a variey of conditions, including trisomy 21 and many forms of intellectual disability. It typically occurs as "overflow, " when the child is excited or frustrated. It is not dangerous, per se; if it occurs so persistently that it interferes with more desirable activities, antipsychotics sometimes are used to diminish the frequency; behavioral approaches may work too.