Add position: absolute; width: 100%; in the container like that:
<div id="sideBar">
<div id="sideHead">TICKET MENU</div>
<hr id="sideHeadSeperator">
<div class="sideItemContainer" id="parentSidebar" style="position:absolute;width:100%; ">
<div class="sideItem sidebarMover" name="football">Football<i class="fa fa-caret-right"></i></div>
<div class="sideItem sidebarMover" name="tennis">Tennis<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Rugby<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Basketball<i class="fa fa-caret-right"></i></div>
</div>
<div class="sideItemContainer" id="footballSidebar" style="display: none;position:absolute; ;width:100%;" >
<div class="sideItem">Arsenal<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Chelsea<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Liverpool<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Manchester Utd<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Manchester City<i class="fa fa-caret-right"></i></div>
<div class="goBack"><i class="fa fa-caret-left fa-inverse"></i> go back</div>
</div>
<div class="sideItemContainer" id="tennisSidebar" style="display: none;">
<div class="sideItem">Wimbledon<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Rome Masters<i class="fa fa-caret-right"></i></div>
<div class="sideItem">US Open<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Roland Garros<i class="fa fa-caret-right"></i></div>
<div class="sideItem">Madrid Masters<i class="fa fa-caret-right"></i></div>
<div class="goBack"><i class="fa fa-caret-left fa-inverse"></i> go back</div>
</div>
</div>