medical resons is best for your case

America is a very big country.   Tell them that your mom would like to see other parts of America also. Submit  a bank statement for a few thousand dollar to convince them that you have enough fund for travel.  That should be enough for them to exnend your mom's visitor visa.

Tom bro
Is this her first extension ? If yes, there won't be any problem for first time visa extenders. As rebelcmx said you can give the reason of more travel time within the US or have someone in US send her some kind of invitation to attend seome events ( marriage, bratabandha etc.), which is sufficient as far as I know.

As earlier stated by former posts medical reason visiting different parts of US babysitting for grand kids might be one of the many reasons for extension of stay.Of course you can back those extension by supporting docs like recent paystubs W2 fat bank account will be good documents for evidence.

I'm not sure if it's a good idea to mention about medical reasons.. I wouldn't make the case more complicated. Visiting different parts of the country and celebrate festivals (Dashian/Tihar/Christmas) together saying you haven't done that for a while together as a family would be sufficient.

Guys, I also wanted to extend the b2 visa for my parents. I had following questions. I would really appreciate if you guys could answer these.
1) Should I file it online or postal?
2) Can I file it for my parents via online? Am I considered the 'representative who is authorized by USCIS or the BIA to represent USCIS customer' as described in USCIS ELIS Online registration page?
3) Can I use my credit card to make the payment for the filing fee for my parents if the USCIS ELIS account type is 'applicant'?
4) How many days do they have to leave the country if it comes denied after their current I-94 is expired?
5) Will that be counted as overstay when they want to revisit US in the future?

b2 extension is very dangerous