Goose.....I am very familair with the place. I only live about 2 hours away and take my son there 2-3 times a year. We go for about a week during the summer then usually get down there during the off season a few times just to get away. Anyway....here's some info for you.
With a 6 year old, you would be better off staying on the boardwalk if you go. The way OC is set up is that it runs about 140 streets - one long strip from the inlet north almost into Delaware. The boardwalk area starts at the inlet and goes up to around 26th street. In the summer months, this place is packed with families with kids since it's full of shops, eateries, arcades and 2 sets of rides down by the inlet. There is one other amusement park around 33rd street which is good for kids 7-under but its only open during the summer months.
The rides wont be open in early April but the arcades are open. Not sure what kind of hotel prices are in your range, but I always stay at the Holiday Inn on 17th street. They have an indoor pool and every room has a jacuzzi hot tub. It's expensive as shit in the summer (about $400 a night - but if you see the outdoor pool for kids you'll know why people pay it) but in the off-season I would guess you'd be paying $130-$150 a night. You can google this and get more info on the place.
Just outside of OC there are go kart places but not sure how many are open in off season. My son isnt into them yet. Rt 50 comes into OC around 4th street near the inlet. On 50 just a few miles outside of OC is where you can find the go kart places and probably a few laser tags places and putt putt. Inside of OC there is putt putt on almost every street but alot of them close down in off season.
If you're driving from Cleveland you're talking about a 7 hour drive. You better make it a long weekend (Fri-Mon). Some hotels have more to do for kids where you might get an indoor pool and even a small game room. Most of the hotels down on the boardwalk are older and dont have these modern features. There are nice hotels all over though. I prefer to stay on the boardwalk, especially in summer, cuz driving is crazy during the summer, plus I like to walk the boardwalk. You can also rent bikes in the offseason to ride up and down the 5 mile boardwalk.
When I go there during the off-season we spend some time around the hotel, some time down on the beach playing football or frisbee, then some down on the boardwalk hitting the arcades. The arcades will empty a wallet in a hurry. Most games are $1 although you can find some for less, but they are few and far between.
For the distance you'd be coming, I am not sure there is enough to keep YOU happy and supplied with money. If you're willing to spend spend spend, and your son is like mine and a video junkie, then he will have a good time with the arcades.......but you might get tired of the place in the off-season for 4 days. Alot could depend what kind of hotel you choose as to how much you like it.
The place gets even more expensive in the summer. It's Maryland's primary beach resort and it's absolutely nuts there in the summer. I go for a week in July and will probably make my hotel reservation in early February.
If you have any other questions, let me know.