One thing I would say about Legoland is that the hill the park is set on can make it quite a trek to negotiate at times if you’re not the most able walker. I’m a relatively adept walker (if I do say so myself!), so I didn’t struggle with it too much on my most recent visit, but I have been in the past with a member of our party being in a wheelchair and it was quite hard work! With a small child, I can imagine that their littler legs might potentially struggle. So I’d recommend making use of the Hill Train, if it’s open.
I’d concur with @JoshC. on the note of queues. I found many queues to be pretty overstated on my last visit, and like Josh, I also found Minifigure Speedway’s queue to move quite quickly compared to that of other rides.
On the subject of Minifigure Speedway, I should also warn you that they do not let you pick your side as they do on many other duelling coasters. I struck lucky and managed to get on both sides without needing to ask, but if you are keen to try both, you may have to ask.
I haven’t visited with a young child (other than when me and my older sister were young children ourselves on our initial visit in 2009, which is now some years ago!), so I apologise if I’m not too useful to advise you on visiting with a child, but I hope you have a nice time regardless!