$20,000 a month isn't exactly low... but then again, as a percentage of her income (if that's accurate) it's pretty abysmal; it would need to be increased to $30,000 to even be 1% of her earnings... and for two children, closer to 5% seems much more realistic? I remember when Poor ol' Eddie Murphy was paying Scary Spice something like $50,000 a month and that was for ONE child.
On the other hand they should surely be relatively comfortable on $240,000 a year + whatever his earnings are