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My Non-Professional take…
Get your chord transitions down first. You will always lag or lead the beat until you can move from chord to chord fluidly enough. If you are picking (flat or finger), you also have to develop the timing between your two hands. Before that working on rhythm is like trying to walk and run all in one go.
Once you can get the basic mechanics of just playing the instrument down, then you can turn to a metronome if you want. You can also use backing tracks, the actual song, or a drum track to work on timing. When you do start to learn rhythm and timing, start with making sure you can do a down-strum on each beat, then learn to do a down on the beat, and an up on the “&” count. From there you can then start to learn strumming patterns. There are good resources in the skills section to help with this.
Other more experienced guitarists may chime in with other or better advice, but this is how I go about it with my kids.
Just a fun glimpse into my guitar life with kids…
I am in a position where neither of my kids wants to use the annoying thing. Instead, I find myself, a couple of times a week, just being the metronome for them by chunking muted strings for them in a steady beat (while trying to ignore and pay attention to what they are doing simultaneously so that I don’t speed up or slow down). All smiles and encouragement for them while I die a little inside lol.