Kiddy Infinity Pro - when should the impact cushion be removed?

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Hello, we have a Kiddy Infinity Pro for our son who is 4 years old and quite tall, but not very heavy for his age.

When should the impact cushion be removed? I have had a look on the Kiddy website but am struggling to find any conclusive info.
Also is he ok to stay in the seat until his head is above the top?

Many thanks.

We have the same model for our just turned 4 yr old ds. He hasn't had his cushion on for about a year iirc. He is 110cms and around 20 kilos so big for his age. He was probably just about 18 kilos when we took it away.

2.5 yr old dd is 90 cms and 13 kilos. She still has her cushion. No plans to remove it anytime soon.

Was thinking I needed to look at the instructions soon though as I can't remember what it recommends.

The infinity pro should never be used without the impact sheid as it is a 9-18kg stage 1 seat so the same as a 5point harness only seat. It is not tested to be used without the sheild so would be dangerous to do so as the seat may well not protect child in an accident as it was not designed to be used in this way so belt may not line up properly or stay in place accross childs body.

The gardian seat however is a stage 1-3 seat and the sheild is used between 9-18kg then can be used ad a high back booster by removing the sheild. You can remove sheild at minimum of 15kg but it is recomended to use it until 18kg.