I agree with what others' have posted... Two things that stood out most were the spin class and Christmas is coming...
I love having a trainer, without it I wouldn't get to ride. I have a (near) two year old and 5 year old, so the only time i can ride is during naptime/kindergarten on the trainer in our garage. I would definitely communicate how much you want to ride to your Husband and every time he asks what you want for christmas, say a trainer or money for a trianer. and if he doesn't ask what you want for Christmas, make sure he knows.
Until Christmas, I would check out the spin classes at your gym. I have trouble taking my kids to the gym kidsclub because they always end up sick, but if your youngest is 4 you might not have that problem. I've enjoyed spinning classes in the past and hope to make it to some this winter. They are great for cardio. But the trainer is definitely more convenient with kids.
I have a double trailer for my two kids and that works some of the time. I have to be careful with my knees though pulling that much weight and hills are really tough. How big is your 4year old? And if you don't already have a trailer or tag-along, a trainer is actually more economical. I don't make it out on the road much right now... maybe when the kids are both in school full time. I still go back to the trainer being the best bet for you right now if you can get your hands on one.
Good luck!