Barcelona Viator Horseback tour

Rather than use Viator, which is only a re-seller of other company tours, check Trip Advisor for Barcelona and I bet you will find this same tour offered there. Then you can book with them personally.
Sounds like a lovely and memorable adventure and a good use of time. I would do it. Your girls are going to remember this more than visiting any church or museum.

Some more info...
http://www.amatluxury.com/blog/collserola-a-reference-of-horse-riding/
http://barcelona-home.com/blog/horse-riding-barcelona/
http://www.hipicamasbatllo.com/en/turismo-ecuestre/

For some background on that famous "natural park" which is never mentioned in Viator's website.... it's called Parc de Collserola and it's located on the ridge north to the city with the same name, the same one which also has the Tibidabo mountain and its famous fun fair overlooking the city with magnificent views.

The park is considered to be the biggest urban park in Europe, it's an 8000-hectare protected park, 22 times larger than Central Park in NY or 8 times larger than Bois de Boulogne in Paris. The Mediterranean climate and varied relief of the land makes the park a thriving site for mixed woodland, with many examples of white pines and evergreen oaks, as well as riverside copses which alternate with farmland and brush. The park is home to some 190 species of vertebrates, including squirrels, foxes and the occasional wild boar, as well as a wide variety of bird life. The principal point of interest is the sprawling Museu-Casa Verdaguer. Catalonia’s revered writer Jacint Verdaguer lived in this late-18th-century country house before his death on 10 July 1902. Beyond, the park has various other minor highlights, including a smattering of country chapels (some Romanesque), the ragged ruins of the 14th-century Castellciuro castle in the west, various lookout points and, to the north, the 15th-century Can Coll , a grand farmhouse. It’s used as an environmental education centre where you can see how richer farmers lived around the 17th to 19th centuries.

Yet, it's indeed a shame you "loose" half a day of the otherwise very short visit to Barcelona... :)))

Viator is a Trip Advisor company and when you look at their listings most of them link directly with Viator.

This is true what UncleGus says about Viator and Trip Advisor, but you don't have to book with them if you go to the company website instead.