An adult white dragon will literally kill all of those characters in a single turn each, if not faster. It can fly and use it's breath weapon (why would it ever land?) so the Rogue's Flametongue is useless. The Moon druid is kinda useless since their focus is on wildshapes instead of spells. The Wizard can magic missile for a whole 15 damage per turn maximum. This will never end well for the party unless you don't play the dragon appropriately.

You are also doing your calculation very wrong... If your group is 2 lvl 5's and 2 lvl 2's (which is already kinda dumb your party should be the same level) the deadly budget is 2600. If it is 4 level 5's that budget is only 4400. A young will present a challenge if played well but would likely still obliterate the party because dragons are ruthless and intelligent. The Adult is a fast track to a single or two turn TPK.