When dealing with a headache, try boiling one cup of apple cider vinegar and one cup of water in a pot. Pour into a heat proof bowl and put on the table or other safe place that you can lean over the bowl and cover your head with a towel. Inhale the steam for a few minutes to bring some relief.
The warning on mine says not to use prior to 4 months of age. My son was in his Jolly Jumper from 3 months. The directions of use are that if they can confidently hold up their head then they are fine.
Carseats and boosters should NEVER be installed in side-facing jump seats. Unfortunately, that means that children under 8 years old (about 80 pounds) should not ride in these types of seats. Children should never ride in the cargo area of a pickup. Rear-facing seats in wagons vary.