To get familiar with how the Online Analysis tool works, I highly recommend watching the Tutorial Video, paying particular attention to the segment on filters. Using age as a filter you can limit the sample population to children. Simply type age(0-18) into the Selection Filters: box. This will give you results for all respondents between the ages of 0 and 18. Since you are interested in comparing males and females you will want to put the sex variable in the Column: box. By default, percentages are calculated for columns, but you can change this in the TABLE OPTIONS: under Percentaging. Depending on what demographic you are looking for, you can either apply more filters (separated from age(0-18) with a single space) to restrict your population further, or you can add a variable as a row. For example if you type race into the Row: box, with sex in the Column: box and age(0-18) as a filter, the resulting table will provide the percentages of male/female children in each race (e.g. in the 2012 ACS, 68.3% of Males 18 or younger are White).
The Instructions Page from IPUMS-USA is also very helpful for understanding how to use the Online Analysis tool.
I hope this helps.