XF 1.5 Adding node_id checkbox to search_bar template (quick search form)

How can i add a checkbox to search_bar template that searches only in multiple specific nodes_id when selected ?
I'd also like to include child forums in search results when this option is checked, if possible (like #ctrl_child_nodes checkbox in full search page)

So far here's what i have:
<input type="checkbox" name="nodes[]" value="11,27,12,40">
The problem is that even if multiple nodes ID are used, it will only search in the first forum (node #11)
The result search URL is using parameter &c[node]=11 even if i searched with parameters nodes. What am i doing wrong ?

Note that I am using Elastic Search, not sure if it changes anything.