Hi Carlos!
Thank you for bringing your question to the Disney Parks Moms Panel!
You are going to love the Walt Disney World Marathon. It's a great course with lots of fun and magic along the way!
The Health and Fitness expo is for everyone. Your family is welcome to join you! The expo tends to be the most crowded early in the morning and is more crowded the first couple of days it's open. If you're not too worried about possibly missing out on a few items of race merchandise, I recommend going Friday afternoon or even Saturday. It's much less crowded then and, in my opinion, much more enjoyable...especially with a family.
You can pre-order certain items during the registration process. So, if you have your heart set on the race pin or jacket, definitely pre-order those as they tend to sell out quickly. I have noticed in the last year, though, that runDisney is doing a better job at holding items back and restocking throughout the expo weekend. I ran the Goofy Challenge this year and didn't go to the expo until Friday afternoon. I found everything I wanted (and more!) and the crowds were light and manageable.
I hope this helps! You can find more information about
Walt Disney World Marathon weekend on the runDisney website, and please let me know if you have any other questions.