I think in the People's Republic of Illinois it is. My husband and I wanted to take my 5 yo nephew to a restaurant in our truck but couldn't. My sister in law said it's because children under 12 have to sit in the back seat, it's the law. Our truck has no back seat.
Actually, if you follow the link I posted you can see for yourself what the laws of your state say about child restraint systems.
I checked and found no mention of pick-ups or back seats for Illinois. I might add that Illinois takes a similar tack that North Carolina does. Namely, that even if you're in violation of their child restraint laws, there's no negligence. They even go on to say that the violation can't be used as evidence in any civil legal proceeding. Note Section 5.
Here's what Illinois has to say about child restraints:
Illinois (625 ILCS 25/)
(625 ILCS 25/1) Sec. 1. Title and citation. This Act shall be known and may be cited as the "Child Passenger Protection Act".
(Source: P.A. 83-8.)
(625 ILCS 25/2)
Sec. 2. Legislative Finding - Purpose. The General Assembly finds that a substantial number of passengers under the age of 6 years riding in motor vehicles, which are most frequently operated by a parent, annually die or sustain serious physical injury as a direct result of not being placed in a child passenger restraint system. The General Assembly further finds that the safety of the motoring public is seriously threatened as indicated by the significant number of traffic accidents annually caused, directly or indirectly, by driver distraction or other impairment of driving ability induced by the movement or actions of unrestrained passengers under the age of 6 years. It is the purpose of this Act to further protect the health, safety and welfare of motor vehicle passengers under the age of 6 years and the motoring public through the proper utilization of approved child restraint systems. (Source: P.A. 83-8.)
(625 ILCS 25/3)
Sec. 3. Definitions. The terms "highway", "motor vehicle", "owner", "police officer", "recreational vehicle", "roadway" and "street" as used in this Act, unless the context otherwise requires, have the meaning ascribed to them in The Illinois Vehicle Code, as now or hereafter amended. For the purpose of this Act, "motor vehicle" does not include motorcycles. (Source: P.A. 83-8.)
(625 ILCS 25/4)
Sec. 4. When any person is transporting a child in this State under the age of 4 years in a non-commercial motor vehicle of the first division, a motor vehicle of the second division with a gross vehicle weight rating of 9,000 pounds or less, or a recreational vehicle on the roadways, streets or highways of this State, such person shall be responsible for providing for the protection of such child by properly securing him or her in a child restraint system. The parent or legal guardian of a child under the age of 4 years shall provide a child restraint system to any person who transports his or her child. Any person who transports the child of another shall not be in violation of this Section unless a child restraint system was provided by the parent or legal guardian but not used to transport the child. For purposes of this Section and Section 4a, "child restraint system" means any device which meets the standards of the United States Department of Transportation designed to restrain, seat or position children. (Source: P.A. 88-17.)
(625 ILCS 25/4a)
Sec. 4a. Every person, when transporting a child 4 years of age or older but under the age of 16, as provided in Section 4 of this Act, shall be responsible for securing that child in either a child restraint system or seat belts. (Source: P.A. 92-171, eff. 1-1-02.)
(625 ILCS 25/4b)
Sec. 4b. Children 6 years of age or older but under the age of 18; seat belts. Every person under the age of 18 years, when transporting a child 6 years of age or older but under the age of 18 years, as provided in Section 4 of this Act, shall be responsible for securing that child in a properly adjusted and fastened seat safety belt. (Source: P.A. 90-369, eff. 1-1-98.)
(625 ILCS 25/5)
Sec. 5. In no event shall a person's failure to secure a child under 6 years of age in an approved child restraint system or properly secure such child, if age 4 or 5, in a seat belt constitute contributory negligence or be admissible as evidence in the trial of any civil action. (Source: P.A. 86-1241.)
(625 ILCS 25/6)
Sec. 6. A violation of this Act is a petty offense punishable by a fine of not more than $50 waived upon proof of possession of an approved child restraint system as defined under this Act. A subsequent violation of this Act is a petty offense punishable by a fine of not more than $100. (Source: P.A. 92-173, eff. 1-1-02.)