Lightyear 2022 is a prequel to the Toy Story saga. It attempts to explain Buzz's backstory. Buzz is voiced by Chris Evans aka Captain America and so Buzz already has that national hero, courageous, selfless "tone". Basically Buzz crashes his spacecraft and finds himself and much of the Star Command stranded on an alien planet. He has to find an energy soure strong enough to take everyone home.
This movie is what Andy watched that first got him interested in Buzz so from that point of view it's an interesting extension of the Toy Story franchise.
While it is deemed to be suitable for children above 6+, there is some violence towards the end where Buzz and crew have to fight their way off the planet. Most of the fighting scenes are battles against intergalactic species and are filmed from the point of view of playing a video game so are pretty cool scenes.
No gross violence as such. Zurg can get a bit scary but again nothing too unexpected - we have known of him and been prepared for him since Buzz joined the toys at Andy's. There is one gay character an a brief same sex kiss. This recieved a lot international backlash. I personally believe that it's great that Disney introduced diversity - it will encourage open mindedness from an early. However if you feel it will confuse your children prepare them with a talk beforehand. Each family is different so you'll have to handle this - note that this is Disney and there will be more such scenes going forward so it's best that your kids are aware and open to this.
With a run time of just under 2 h, some critics have complained that Lightyear was quite boring in the last 45 m. It has been termed mediocre and "not funny" by many. I think it will depend very much on how much your child and whether they can sit through a movie that may not terribly riveting right through.
For yourself, I would buy a coffee - you might need it.