Once the assessment is done, some links give you two weeks free trial, other links give you 5 lessons free trial.
I also recommend you do the replays (this is in addition to revision in bedroom). On this, you can improve your time, and compete with other people to get better times. You get bronze, silver and gold awards depending on your results as well. They show the time it took, the target time it should take, and you can replay as many times as you want.
our mistake was my kids didnt do enough of the replays, and sometimes they can do the exercise because they were told how to do it at the beginning, but then dont remember what they did. So practicing and replaying is really good to do.
There are other online options available, at cheaper prices, but I do like that you can keep a record of how they actually did, whereas free sites you have no idea how they coped with a topic, unless you sit with them all the time.
There is Mathletics which is also quite a good package, but for struggling children, I find the teaching of strategies very unclear. With Whizz, if you get an answer wrong, it takes you through step by step to work out the answer. With Mathletics it just tells you the answer, and then its up to you to look at the help to see what you should have done (most kids just move on).
I am very positive about Whizz, and believe it does help - if your child likes computer programmes - but there will be plenty of people on here who say you shouldnt need to help your kids at home.
One downside is if a child cant read the questions, there is no audio to narrate the questions.