Huertas is a great location to visit the Prado where currently there is an exhibition of armoury-the rest of it is in the Royal Palace.Close to the Prado is a great Naval museum.
I suggest you visit the tourist office in the Plaza Mayor where you can choose tours for under 5 euros!There is a booklet for children to work through the main sites of Madrid and when they hand it in they will receive a very cute Madrid bandana and pencilIf you get an early start,make a trip to Segovia,your son wil love the aqueduct and castle!
In the Huertas area,I have stayed at the Jaen-lovely couple but speak very little English.There is a plaque showing the birthplace of Cervantes and further down the same pedestrian street is the Dulcinea-very clean.The house of Lope de Vega is on the same street and it is free,open to the public.
If the weather is bad,then the on/off bus would be great to get an overview of Madrid.
Madrid is a very kid friendly city!