I have a cherished Samsonite case which has been my companion for the past decade and it conforms to all the European Low Cost carriers' required dimensions - even Ryanair when I leave the front pockets empty.

However, I just returned from a short trip to Tampere, Finland with my 3 year old daughter and realised that none of our other bags are within the strict Ryanair 55x40x20cm requirement. I ended up having to make up a bag using a strong square shopping bag and compressing the clothes with a large furoshiki square of material. Then some light plastic boards and a luggage strap to stop the sides from bulging (the weather was -24C in Finland and I had no intention of doubling the cost of my flight to pay for check in luggage).

The short story is that this last trip demonstrated that LapChild needs her very own case. One that she can use (4 wheels are ideal for this), one that isn't too heavy (around 2.5kg being the max), and one that she can love and cherish as her own travel companion.

I looked around, and searched and hunted, and then came across SanDiego1K's original post also. Started investigating Lipault and, result!, Lipault have a sale on their 4 wheel 55cm cabin size... in fuschia pink, my daughter's favourite colour.
Declared dimensions are 55x35x20cm and it weighs 2.7kgs. Practically perfect in every way.

EXCEPT...

Bag arrived. My daughter is thrilled. I'm concerned. Take out the ruler and, ouch! The bag is actually 23.5cm wide instead of 20cm. (The plastic housing the wheels makes the bag stick out way more than the declared 20cm for at least a third of its length).

In summary - this case can't be used with Ryanair without risking a £50 fine.

I've contacted Lipault who agreed verbally that the dimensions stated are an error.
I accept that the actual measurements are within IATA guidelines (and can confirm that it falls under the 25cm requirement) but this wasn't the reason I chose this particular case.
Have also sent an email as suggested requesting that they send me the stickers needed to return the case to Paris and Lipault will then issue a refund.

A real shame as it was ideal in so many other ways.

For the record the actual dimensions are 55x37.5x23.5cm
The 37.5cm width measurement can expand out a bit but it should be easy to keep it under the 40cm guidelines for other carriers.

Hopefully, Lipault will do as they said they would and amend their website descriptions as this error has been a real nuisance.