Yes, you are right.
The child one theme does not have mobile logo and alternative logos in their header.
So you cannot add alternate and mobile logos for it.
Go to wp-admin->appearance->customiser->add custom CSS-> add these lines and save.
I found out that there is some issue with your mobile header toot, This css will fix your current mobile header.
@media (max-width: 768px)
{#logo img {width:180px !Important;}
#trigger{right:20px !important; margin-top:5px !important; }
.topmenu {
margin-right: 5px !important;
padding-top:0px !important;margin-top:-60px !Important;
}}
div#vibe_bp_login:after{left:100px !Important;right:130px !important;}
.