Just like tears, laughter is also an opportunity for stress release. Silliness together is often a great way to reduce the stress load for your child (and you). Ensuring that what you are doing is something your child wants to do is the key. One thing to be very cautious about is tickling. Many children don't like it but can't help but laugh, and then are helpless to stop you. We would advise that you only tickle if your child wants to and likes it.
Creating some time to laugh, play, run and chase and just follow along with whatever your child wants to do on a semi-regular basis can do wonders for your child and their emotional well-being. We call this special time and the only rule is that it's you and your child doing what your child wants to do (they are in charge, within safety limits) for a limited time (we say 10 to 30 minutes). It's even better if there is a time during the day that your child is anticipating this and knows when it's coming again. If you have more than one child, finding a time for this with each child is especially helpful. You can find out more about this method on the handinhandparenting.org website.
One tool that we find helpful for children to process unusually upsetting events is empathy books. These are short narrative books made with paper and pen that may include some drawings of what happened and some brief text that the child helps you with, and captures the child's experience. You can create these with your child's input and words and they can be revisited when your child is remembering the event, helping them to get perspective and process. This is not a story with a moral or a story that has a point, it is simply about what the child experienced and how they felt when it happened. If your child puts their own happy ending on the story, then that is helpful, but if they don't they aren't there yet and they just need to be sad or upset until they feel resolved about this. Trust their process and they will be given a gift like no other in this life.