Hi Villenas, although April is not in the rain season of Beijing, it is suggested that you should take rain gear just in case.
It is impossible for you to finish visiting the Great Wall and the Tiananmen Square (this is an essence of Beijing that almost could cost a whole day) in half a day. The great wall is in the suburban area, so if you leave for the wall in the morning by yourself, you could come back to the city almost in the afternoon. This day's long excursion may be a little tiring, especially with little kids. In this case, you could come to the Square at night but all of the must-sees there are closed then. In my opinion, you'd better allocate more time for these two sites.