Different counties handle charges differently, but in my experience district attorneys want proof that you place your child in the proper restraint while in your vehicle. I generally advise my clients to provide me with a picture of her child strapped into a car seat, or for older children, a picture of the child wearing a seatbelt. If you show this proof to the district attorney on your court date, he or she may dismiss your case, and if you case is dismissed you won't have to pay a fine or court costs.
If you've already plead guilty there is not much that can done. If not you can ask the DA to dismiss the case but this is unlikely. In that event you can ask the judge for time to pay or continue your case for a short period so you can cone up with fnds. Some judges will put you on probation until you can pay but this will also mean more costs for probation.