The founatain you're referring to
must be the cascading water at the base of the
Plaza de Colón . Here, you can walk down some steps and walk under the water. It's really pretty cool - both figuratively and literally speaking. Plaza de Colón is the first plaza north of fountain of Cibeles..
El Teleférico is, in my opinion, a nice and high 15-minute ride on a 2-4 person gondola from/to Rosaleda Park to/from the large Casa de Campo. It gives a good view of the western side of Madrid while going over the river valley. I like this ride. They give, in Spanish, an audio description of that which you're seeing from that particular position on the "ride". On the Casa de Campo side they have a high terraza that serves snack foods and drink with a decent, distant view of Madrid. Keep in mind that some have been bothered by the obvious prostitution presence in the Casa de Campo in the day and night.
Other ideas:
Madrid\'s Zoo is supposed to be good as is the
Parque de Atracciones (Amusement Park). Both are located in the Casa de Campo.
Not far from the Rosaleda Park is "El Faro de Moncloa" (the Moncloa Lighthouse) which is a 92-meter half-disc, floating spaceship on a 92-meter pedestal and gives a good 360º view of western Madrid. It's cheap to go up but is closed on Mondays/winters. You go up/down in a glass elevator and is never busy. It's located by the Arco de la Vitoria.
a
row-boat ride through the ducks on the "lake" of the Retiro Park always looks like so much fun. I've never done it but always wanted to. I understand it's a bit pricey for the 15-30 minute boat rental.
Also in Retiro Park you'll find many street performers and puppet shows. But, of course, he won't understand (will he?) Spanish so the puppet shows won't be so enjoyable.
If he's into gory stuff (as are many 10-year olds) and isn't prone to nightmares you could consider taking him to the bullfight museum in the Plaza de Toros - Las Ventas.
That's all I can think of this morning. Hope you both have a wonderful time. There's SO MUCH to see and do in Madrid it's incredible.
Saludos, MadridMan