Can you tell us a little more about what the students have difficulty understanding? If you have any specific examples that have been particularly difficult for the students, we can take a look at the language and see whether there's anything we can reword.
I don't know of any resources that would be appropriate for young Spanish speakers learning HTML. I think that it will be difficult for them to understand the level of Spanish, especially the technical aspects of it, if they look at materials for adults, and they wouldn't be able to ask you about what they were reading, unless you also speak Spanish.
There are a few ways that teachers have mentioned supporting students in getting the English they need to succeed in the class. The one that's been promoted the most to me is introducing key vocabulary to students before starting the main lesson. Teachers have also mentioned pairing students, either in same or cross language groups, so that they have the opportunity to speak and listen about what they are learning. In general, making sure that information was presented both orally and visually, including keeping the (English) subtitles on videos, has been really helpful for my students in the past. I've also had to stop videos frequently to give students a chance to absorb information.
We want CSD to be accessible to your students, so please feel free to give feedback on any language that the students found particularly challenging, and we can look at it again when we do the next round of revisions on the course.