what is the problem with a sloping yard? drainage issues? how do you plan to use the yard?

Great feedback @ ASVIneriors
@DIAspoton: The problem is the slop is deep makes it dangerous for little kids and it is unattractive.

A Landscape architect can give you solutions - bear in mind you are not obliged to buy everything; they can give you a plan with planting advice that you can do yourself at your leisure, or they can take on part or all of the work. Seriously you will be well served dealing with a professional in this matter. All the best

okay, got it. i agree with ASV. consult a landscape architect.

it looks like you have a fair amount of level space for the kids? if so you could run a welded wire (unobtrusive) fence across at the point where the slope begins. plant a mix of trees behind the fence to create a woodland effect and act as a sound buffer. please don't plant a row of trees. nothing natural about it.

Does the sloped part go down the back of the property or down the side of the property?

Do you have any sort of build line? We were planning to do something similar at our old house but it turns out that there was a city build line that made it not possible. I would also second that you really need to think about drainage. Right now the yard drains away from you so whoever or whatever is behind you is getting any potentially problematic drainage you have. How much rain do you get? If you level out that yard the rain will need to go somewhere so any plan you have need to include a drainage plan if you do get any significant rainfall.