Being a passanger on a cargo ship?

I haven't ever been a passenger on a cargo ship. I doubt that they would want children on board. I travel with my disabled three year old son and baby daughter that is one year old. If it was possible I would like to travel From Miami to somewhere in Central America like a port in Costa Rica. When I was at college I got to spend a week on a training vessel. It went down the River Thames to Essex stopping off at the Med Way. It wasn't comfortable on board and night watches were required. It was an interesting and worthwhile experience. I was in Cape Town and I met some people that were cruising around the world. Due to bad weather they missed several of their expected destinations in the Atlantic Sea. So sometimes there can be high waves and a rough sea.