My sister has an 8wo and has been using a door bouncer for about 2 weeks now! The age to begin using these as per the one i have is 4 months or when the child can support their own head, DN's head doesn't wobble and can hold it steady for a while but does this mean that she can use one of these? I don't want to say anything and worry my sister unnecessarily or sound like I'm preaching but if using this could cause any harm to DN then obviously I guess i should say something! Can someone help please!!! Also is there any harm in letting a baby sleep in their car seat all the time?