It would count as your DD's one item of hand luggage (assuming it is within the size restrictions for hand luggage). She would not be able to take another bag as hand luggage as well, as I understand it. More than likely they would make you check in one of the two items, especially when the flights are very busy.
smile - you can take it as hand luggage (and i have done) but the person carrying it as their hand luggage almost certainly won't be able to take another bag as hand luggage too. We didn't try, so I didn't actually test taking a second one, but the check in person did say "that counts as your piece of hand luggage". We were on a full flight.
Basically you cannot assume or expect to have the booster plus any other bag IME. Year before last, they even insisted that the DC's teddies be put inside their hand luggage and not be carried as separate items!
The only time I've ever seen Easyjet be a bit lenient on the hand luggage thing is on emptier flights.... mostly they are pretty strict especially on busy flights.
we have just come back and placed the seat in the handluggage.. as a result, our hold luggage was over (about 2kgs per suitcase).. they didn't charge us extra for this.. once again tho it may well be dependent on how full flight was... which you wouldnt know in advance.
We have just flown easy jet and we found the booster seat fitted in one of those sturdier reusable super Market bags fine, along with a couple of books and snacks. My dh carried it on as his hand luggage. You can also buy a booster seat/back pack thing from the people who makes trunki's which is about £35 which was our other option because that would then be your child's hand luggage and booster in one.
To answer your op the booster seat if you carried it separately counts as your hand luggage and you can't take anything else on board easyjet.