Joaquín Sorolla Museum, Madrid (for Katriona Fahey)   Teacher looks too young for motherhood but is in full command today of 15 four-to-fives who have heard her say “No tocar, no tocar” so many times the uniformed attendants need only smile upon her brood, barely beyond milk mew, too young to be smart, hand-holding twixt rooms, two-by-two, their introduction to art cross-legged on the wooden floors of the home of Joaquín Sorolla, the very best painter of beaches. His mansion, given to the state in 1930, a leafy respite in the city´s posher reaches, Calle General Martínez Campos.   Jeans,…