Just been helping my sis-in-law on the hunt for art classes for my niece so hopefully i can help you out here.
I'm not too sure about Global Arts cos i've heard varied comments on it so i guess its up to individuals. Most importantly is that your kids enjoyed the environment and teaching methods offered by the center and I guess that you will never know until you sign them up for a trial.
The studios i've looked at are those around Serangoon/Hougang areas which I hope is not too far for you.
My sis-in-law went to visit my art studio and Art Speaks Studio both at Serangoon Garden area and Little Designers at Kovan but finally deciding on Art Speaks Studio cos my niece seems to enjoy it.
If the location are ok for you, you may want to call them up: