There were many children in attendance at the Boston Symphony Orchestra’s first Saturday evening concert of 2018. The performance featured two key elements that could have been of special interest to them — or their parents. The soloist was the deft 25-year-old pianist Benjamin Grosvenor in Mozart’s Piano Concerto No. 21 in C, one of the composer’s most personable works. And the second half was Beethoven’s Symphony No. 5. Those quintessential four notes and what follows are pretty much synonymous with the word “symphony” in the public imagination. If you need further proof of that, just look at the name above the Symphony Hall stage.