Pack snacks and lunch, just in case. Bring money for dinner on the way back home. Arrange for a tour guide 2-7 days in advance. This is an optional multi-day experience; for more adventurous families, stay overnight in a hostel and hike Nevado de Colima (a neighboring extinct volcano) the next day.
Check for volcano activity; you may luck out with heavier plumes of smoke and even lava flow. Feed your guides during your trip; they will appreciate it.
Weekdays will be less crowded, but fewer restaurants will be open on a Monday or Tuesday. If going on a weekend, make reservations for a guide and hostel well in advance. Go earlier in the day, since there will be less cloud cover over the volcano in the morning. However, if the volcano is more active during your visit, a nighttime excursion will let you see the glow of the lava from a distance.
The ideal time to visit is between mid-November and April, before it gets unbearably hot and humid. Avoid the rainy season (June to November).
Email Edgard (the guide we used) at firstname.lastname@example.org and mention A Family Afloat for a discount. You can also organize a large group with a rental van to split up the cost of the guide. Or, to save on transportation, take the bus or rent a car from your location, stay the night at the El Litchi Hostal in Colima and have the hostel organize a guide.