I'd like to find a place South of Boston where my kids and I can go
horseback riding, renting horses and going with a guide.

Hello and thanks for the question!

The following is a list of stables that offer leasing and riding south
of Boston:

InStride Equestrian Center
http://www.instride.50megs.com/

"A Full service Show Barn offering AFFORDABLE Full/Rough
Board/Training/Private Lessons/Leasing/Coaching/Showing/Camps/Clinics
and more!"

Volo Farm
http://www.volofarm.com 

Volo Farm is a "full-service facility offering training board, riding
instruction, summer camps, on-site horse shows and an extensive show
schedule at East Coast hunter jumper shows and dressage shows."

Glen Maura Farms
http://www.arriveonline.com/GMF/farm.asp

This farm has many trails and offers training, coaching and boarding.
They claim to "have horses to suit all riding levels, from beginners
right up to advanced."

Briggs Stable in Hanover, MA
http://www.briggsstable.com/

This farm has over 70 horses and offers individual and group lessons.

Harmony Horse Stables in Littleton, MA
http://www.harmonyhorsestables.com/

This stable is west of Boston, however they have won awards and offer
leasing and boarding.

Boston Equestrian Center in North Oxford, MA
http://www.bostonequestriancenter.com/boarding.htm#Facilities]

This facility provides training, lessons as well as beach rides on
Cape Cod!

Additional stables that I found without websites include:

Saguaro Stables in Bourne, MA
508-759-2544 

High Rock Stables in Walpole, MA
508-660-2345 

Additional Links:

This site contains a bulletin board on which users discuss horse farms
in the Boston area:

DigitalCity.com
http://www.digitalcity.com/boston/entertainment/reportpost.adp?aid=1649&page=article.adp

Additional stables in the Boston area can be found online in the
yellow pages at:

SuperPages.com
http://phonebook.superpages.com/yellowpages/C-Stables/PI-58304/S-MA/T-Boston/

Search Terms:

boston horseback-riding
horseback-riding boston area
boston equestrian 

If you are unsatisfied with any part of this answer or would like
additional information, please request clarification before rating
this answer.

Happy riding!

-blinkwilliams

Thanks for the awesome, detailed answer. I spent an hour on the phone
today, calling local town halls, and only found a few stables nearby:
http://www.briggsstable.com in Hanover;
http://www.bluehillsridingcenter.com/ in Quincy; and Blazing Saddles
in Randolph (781) 961-9132, 384 High St)