Have now looked at Viator tour and here are a few thoughts:
Poble Espanyol is on Montjuic, 50 and bus turistic stop outside.While I am a fan it is not my idea of a MUST for the middle of the day if you have only one day in BCN. As it opens till really late my suggestion for that wd be to do it late in the day if you still have the energy (for supper maybe).
"Your tour begins by visiting the Plaza de Cataluna, Via Layetana, the Cathedral and its cloister. You'll tour the Gothic Quarter with its evidence of the Roman and Medieval periods" That you wd appreciate more just wandering on foot, I feel. The danger of doing stuff with a guided group is that you somehow cut yourselves off from the surroundings, don't get a chance to soak up the atmosphere.
" Next you'll see the drive along the Paseo Colon past the Monument of Christopher Colombus and up to the top of Montjuich from where you'll enjoy a panoramic view of the city, the harbor, and the Olympic complex" This is at Miramar near terminus of 50 bus; not mentioned but you can also visit Olympic stadium which is between Miramar and Poble Espanyol.
"In the afternoon the tour will continue along the Paseo de Gracia to see Gaudi's famous apartment buildings including Casa Batllo then visit Gaudi's fabulous Guell Park declared an artistic monument and world heritage site by UNESCO".
You simply drive up Passeig de Gracia: it's much nicer to walk and I wd recommend visiting La Pedrera (casa Milá) which isn't even mentioned here.
"You will then go back to admire the Sagrada Familia (Holy Family) Cathedral, Gaudi's unfinished masterpiece". Admire sounds like you look at the outside, from the coach? At least take time to walk all the way round even if you don't actually go in.
"You'll also visit Barcelona's Monumental Bull Ring and the Picasso Museum which exhibits works from the blue and pink periods of the world-famous painter". Well bullfighting is not part of Catalan culture as it is part of Andalucia culture so again strange vieit if you only have onre day and Picasso Museum won't grab the 6-year-old much. Not sure about the rest of you, but doing it last thing means attention-span is likely to be short for all of you. Museum is excellent for its holdings of Picasso's earliest work (donated by artist and his sister) but otherwise very patchy in coverage of his career.
My suggestion if you do it all on your own (with a good guide book)is:
Walk all the way down Ramblas (not a glimpse of them on this tour) go into Boqueria market, and down to Columbus statue and across to quayside.
Cross to Maremagnum if you want a good look at harbour. Golondrinas boats near Columbus do short harbour-trips. Maybe appeals?
You can then head back to wander round Old Town (Plaça Sant Jaume, cathedral, Plaça del pi a and possibly over to Picasso museum/El Born).
Then the gaudi stuff: go up Passeig de Gracia for Casa Batlóo and Casa Milá/La Pedrera which is great to visit,as there's an AV display on gaudí's work apart from visit to roof and period apartment.
From there you can get 24 bus or taxis to Parc Guell and spend time exploring (it's my son's top recommendation for visitors), then go down to Sagrada Familia.
From there line 2 metro to Passeig de Gracia and you're near the hotel. After a break, if you've got the strength you cd get 50 bus to Montjuic for those views and Poble Espanyol. Magic fountains may be on too. check nearer time.