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My son is getting close to 4 years of age. He's pretty big for his age.

My cousins have some extra booster seats, and we plan on taking some vacations with them next year. Rather than lugging our own booster seat onto the plane, we were hoping to use their seats when riding in their mini-van.

My son is in the appropriate height range found in this answer, and he is close to the weight range.

Assuming that by the time he is four, he is both within the height and weight ranges listed for booster seats, is that all that is needed to ensure that he'll be okay in my cousins' booster seat? They asked if I thought I'd need a seatbelt extender, but I'm not sure what that means. Are they necessary? If so, when? How can I tell?

Can I tell if my son fits in the booster seat safely just by looking at him? I remember seeing a video showing something about the position of the seatbelt relative to his chest, but I can't find the video anymore.

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A seatbelt has to be long enough to reach around the child and all the booster seat and then to the anchor. Some seatbelts are not long enough so an extender is needed. –  DanBeale May 12 at 1:31
    
You also want to make sure child is tall enough that sealt belt goes over should correctly - maybe this is the diagram you are thinking of? –  DanBeale May 12 at 1:34
    
If your cousins can tell you the manufacturer and preferably model of the booster seat in question, most manufacturers have information available both online and in print that shows how a booster seat should be used and fitted, as well as any restrictions on seats where it can be used, child's height and weight ranges, etc. –  Valkyrie May 12 at 11:08
    
@DanBeale yes, I believe that is exactly what I'm looking for. –  Beofett May 12 at 11:16

2 Answers 2

Is it the right type of booster?
First, you need more information about what the brand and model of booster seat they have. A four year old should be in a high-back booster, not a backless/seat-only booster. Also, in my experience, some boosters have a minimum requirement of 30 lb and some have a minimum requirement of 40 lb, which can make a big difference as far as whether a just turned four year old will be the right size and weight for that booster seat. So you definitely want to know what model they have so you can check whether it's appropriate for the size and weight of your child.

Identifying the Seat
In the likely possibility they have no idea what the model is, have them take a couple pictures for you, one of the seat as a whole, and one of any stickers on the bottom/back of the seat. You can look up the model from the manufacturer's website from the info on the stickers, or do a google image search on the brand name and color(s) of the fabric.

Obtain and Read Instruction Manual
Once you've identified the model, it should be fairly trivial to look up the suggested weights and sizes and other information about it's proper use from the instruction manual on the manufacturer's site. Often a height range is specified, but it is more an approximation than exact, since some children are tall in the torso, but other children are tall in the legs. The important part is the seat belt guide can be adjusted to the height of the child's shoulder. The specs on the carseat (check other websites if not on the manufacturer's site) should tell you the height from the base of the seat to the lowest and highest shoulder strap setting. If it doubt after doing your research, go to your favorite retail store and sit your child on a similar booster seat to double-check.

The instruction manual will have other useful information about safe use of the booster, such as making sure the seatbelt is going across their lap/hips, not their belly button, so read the whole manual to feel confident about how the booster should be used.

Skip the Buckle Extender
This is a fairly typical example of a buckle extender:
Buckle Extender
For short term use, such as a vacation, I would not bother with obtaining a seat-belt extender. The purpose of a seat belt extender would be to make it easier to reach the buckle from the booster seat, which might be useful if, say, you had a booster seat squished between two other carseats and the buckle was difficult to reach. These are very specific to particular car make and model (and sometimes which seat position). While the NHTSA does not have any specific information indicating they are unsafe, it does introduce one potential additional point of failure, so I would shy away from using one unless it solves a pressing problem.

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Do I need a seat belt extender?

No. You would only really need this if your car's seat belts aren't long enough to accommodate your son in the booster seat. Otherwise, if it works fine without then do without -- it's one less failure point.

Can I tell if my son fits in the booster seat safely just by looking at him?

No. You can check the operating instructions to see whether there are precise illustrations of where the straps should be (especially in the shoulder/neck area). But if you want to be really safe, visit your region's automobile association and have them look at your son's seated position.

In practicality, if you only need this during a vacation stay, then I'll revise my above answers to "no, and yes".

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