Translation:They want to watch a movie tomorrow.
No one else has mentioned this here, but I think amongst in South East Asian mandarin speakers, you wouldn't tend to hear the 想 come in front of the 明天 but rather the order you put it in, and it wouldn't sound out of place. However, you've been given a great explanation. South East Asian mandarin can sometimes have slightly different grammar (best example I can think of is "你吃先" instead of "你先吃". Similar logic)
Not really about this section but I'm posting here as it's on my mind now, and hope it might help others. Because 想 can mean: want, miss, suppose, believe, think, wish... I find it helpful to define it (想) as "think about/ of". When you want something, you think about it, when you suppose something, you think about it, when you miss someone/ thing... etc.
想, 要 and 想要 can all mean ‘want’ in some sense, but 想 is the least forceful of the three
In many cases, it might be better to think of 想 as being more like ‘would like to’ rather than ‘want’. When used for expressing wants, 想 can only be followed by a verb or verb phrase. In other words, 想 only means ‘want’ when it’s an auxiliary verb.
Notice in each example 想 is followed by a verb. Also look at how 想 does mean ‘want’ in these situations, but it’s not very forceful. If a speaker wants to be very clear and emphatic in expressing ‘want’, they’re more likely to use the much more forceful 要. Because of the difference in how forceful they are, 想 can often be a more polite substitute for 要. For example, when ordering or requesting something, saying 想 is usually more polite than saying 要.
If you haven't studied pinyin and Chinese phonetics specifically, you should take the time to do it, here is a good starting point: https://chinese.yabla.com/chinese-pinyin-chart.php