If it were me I'd book 2 nights. The night before and the night for the day after the park.
If you were driving on the day, what time would you need to be up at? Taking into account time to get yourself ready and ds, packing bags, packing car then driving you'll be a bit tired getting there. You'll also be looking after dd, feeding, bits and pieces around the house. It'll change your mood for the day, then you have to think about and do the drive home, you tired and a tired toddler who will probably sleep in the car.
Or, go evening before, have a nice bit of dinner, bit of TV, early night, get up ( won't be as early as when you tavel from home as you're nearer park) lovely breakfast, nice relaxing shower, getting ready at your own pace, ds will be in the room with you so you're not running all around looking for him. Pack a bag and off you go, nice and relaxed and rested.
For me I'd stay the 2nd night too, it's usually the same price or for just a little more for the 2nd night. Come back from park drop bags, head out to somewhere for some dinner, pick up some drinks and treats maybe a small toy, head back to hotel and veg out. Get some ice from the bar and have some nice cold drinks in the room and stuff your face on chocolate. Next morning, up for lovely breakfast, take time getting ready, drive back home rested and happy.