Welcome to Cruise-Chat. It definitely isn't too early for you to get your cruise booked. You will find the summer time is a peak season and cruise prices will reflect that. The farther in advance you book the better. If rates should fall before your final payment they can be adjusted. Most cruise lines require final payment 60-75 days prior to sailing. If they fall after final payment a cruise line may give a refund or on board credit. It varies between cruise lines and case by case basis.
Most cruise lines have children's programs. However; some cruise lines are better than others for the children's programs. Carnival, Royal Caribbean, and Disney all have excellent programs. You are limited to which cruise lines sail out of New York and Galveston. The lack of competition sailing out of Galveston also is driving the cruise price higher, along with the time of year you want to sail.
It is against the Terms of Service of Cruise-Chat to give out names of Travel Agents. You can check here
for a cruise agency in your area by typing in your zipcode. They can help you decide which ships sailing out of New York and Galveston will best meet your needs.