Just checking if a 5 yo 45lbs in weight would be ready for a booster. She has outgrown her car seat which apparently can hold up to 100lbs but the belts just don't fit any more.
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Many seats that have 100lbs or so weight limits are designed to be booster seats at the higher limits.
Post what brand the seat is and others should be able to offer more help.
Can you share what brand you have? Are you using it in harnessed or booster mode? I'm concerned that what you are using right now may be dangerous or inappropriate for your child. There is no harnessed seat available commercially that harnesses to 100 lbs. If you are using a 40 lb Alpha Omega in harnessed mode, your child will likely be ejected in an accident, and it makes a very, very poor booster.
Booster readiness is about size AND maturity. Your child is physically big enough for a booster, but whether she is mature enough for one is a judgement call that only you can make. If she can sit quietly at an activity without jumping around, if she has a mature attention span, if she follows safety rules without questioning you, she's probably ready for a booster. If not, she needs a seat with a bigger harness.