Deforestation is one of the reasons some deserts have spread so rapidly. trees behave as a shield to that encroachment

I recently saw a video about doing this - apparently the desert is actually a really bad option if we want to combat climate change - the core problem is that the desert currently reflects a lot of light from the sun. If we cover it with forest or whatever, then suddenly we adjust the albedo of the earth and absorb more heat, negating some of the impact. Combined with the energy need to pump (and desalinate for trees - your suggestion of kelp would presumably help there) water, etc... make it not really work.

Source: https://www.youtube.com/watch?v=lfo8XHGFAIQ

I recommend watching the video - it explains it better and goes into detail - it cites its own sources.

I assume trees are good at trapping the energy from sunlight + combining it with c02 and water to produce carbon, oxygen and more water.

So planting trees in deserts with somewhat decent access to underground water tables should help cool the planet and create a great renewable resource right?

I don't think planting trees is considered to be the unquestionably most practical solution. But it's a simple solution that people can do on their own, and it has a lot of other side benefits like habitat, shade, & proven health benefits in humans.