How to have multiple children in multiple branches?

This is a basic way to get the layout in forest (and not an attempt to carefully reproduce your screen shot).

documentclass{article}
usepackage[edges]{forest}

begin{document}
begin{forest}
forked edges,
for tree={
 grow=south,
 draw,
 double,
 minimum width=3em,
 fill=cyan!10,
},
delay={where content={}{draw=none,fill=none,
    append after command={(tikzlastnode.north) edge (tikzlastnode.south)}}{}, 
},
 [A,
  [,phantom]
  [B1]
  [B2]
  [,
   [C1]
   [C2]
   [C3]
   [,]
   [C4]
   [C5]
   [C6]
  ]
  [B3]
  [B4]
  [B5]
 ]
end{forest}
end{document}