I understand that saying no to your children can sometimes be very hard. but if you don’t want your children to go on the internet then you must take the tablet away no matter how hard it is.
if s/he is using the tablet just to play games then you can just get some other game-boy or something similar that does not have internet access (and are much cheaper btw).
if your child needs to use some other program on the tablet then im sure you can find some other device or service that can help you in the same way that does not have internet. -it might be hard to find at first but its out there.