Drayton Manor or Alton Towers

[noenergy]

Which one is suitable for kids aged 5 and 8?

And which would be quieter.

Going midweek the 3rd week of July so hopefully before the schools break up.

Does Drayton Manor have a splash park or water park? As kids are water obsessed, if not is there one in Birmingham or surrounding areas as planning to stay overnight.

That's a tough question.
Drayton has Thomas Land, which is excellent but I'm guessing your DC would find it babyish (I'm a Reception teacher and most of my class find Thomas babyish)! Drayton does have some family rides (but not many) which aren't in Thomas Land but make sure your youngest is at least 1.1m tall, otherwise it wouldn't be worth it. Likewise for Alton Towers. You've either got Cbeebies Land, or the big thrill coasters, but not masses for the inbetweenies!
Alton Towers has a water park unlike Drayton, but Drayton has the zoo. Drayton also has a 4D cinema if it rains, but Alton has the Sea World! As you can see, it's pretty even.
There's an amazing water park called WaterWorld which is in Stoke on Trent, just under an hour from Drayton.
To be honest, if I was taking children this age to a theme park in the Midlands, I'd probably go for Gulliver's Land. It's also got Splashzone, which is indoors. It's cheaper, and you could combine it with a trip to Woburn Safari Park.