The National Fire Protection Association says, "Each year, approximately 2,400 children suffer severe shock and burns when they stick items into the slots of electrical receptacles. It is estimated that there are 6 to 12 child fatalities a year related to electrical shock due to young children placing items into electrical sockets.  According to one recent Anthony Dawes YouTube video, many of these injuries occur to young children--mostly boys age 3 and under!

http://www.youtube.com/watch?v=pxl33R39OV8&annotation_id=annotation_47156&feature=iv

The NFPA also notes that owners of homes and apartments tend to change and no matter whether a family has children or not, new dwellings are required to have the tamper-resistant receptacles in all new dwellings. This change to the National Electrical Code became effective in 2008, and while plastic caps and sliding outlet covers are good, they need to be tamper proof to protect young children especially.

http://www.nfpa.org/itemDetail.asp?categoryID=1508&itemID=36117&URL=Safety%20Information/For%20consumers/Causes/Electrical/Tamper-resistant%20electrical%20receptacles&cookie%5Ftest=1

It should also be noted that exposure to electrical shock and burn accidents are not limited to a child's own home, as children frequently visit homes of relatives and friends who may or may not have children. The National Electrical Code® Article 406.11 requirement for tamper-proof receptacles ensures all new homes and apartments are safer for children, whether they are in their own home or they are visiting other homes.  

So, if you have an older home, at a minimum, to keep young children safe, get the plastic caps or install new sliding outlet covers, but for maximum protection, have your electrician install the tamper-resistant outlets and urge relatives and friends with young children to be aware of this requirement as well.