I have not been in Buenos Aires in 5 years but the Sheraton Libertador is in the centro area.
Very close to calle Florida, a few blocks from the Plaza de Mayo, walking distance to San Telmo and walking distance to the Teatro Colon
You are also close to Galerias Pacifico; nice small shopping mall with great architecture.
I have visited several times and lived there many years ago.
It is a truly awesome city that MUST be enjoyed on foot.
Not sure if you need a guided tour. Just walk from your hotel up to Palermo and its gardens on any of the major streets.
Do not miss Recoleta and the cemetery. It is worth a stop and tour.
Go into the Palace Alvear Hotel and at least have a cup of coffee or a drink. It is - for me - the grandest hotel in Buenos Aires.
Safety I think is always an issue where ever you travel. It ain't no NZ or Australia but it is generally safe. Common sense is your friend.
more than the size of the children, not flaunting large camera, phones or lots of money should be the rule.