My experience has been Myrtle Beach is not in full swing in April. The weather is moderate and warm enough but it is not true beach weather.
Many attractions are closed or not in full swing. One of the big three CG's have a indoor pool. Maybe Lakewood? I would seek out the pool for the kids.
Myrtle Beach is still a worthwhile destination in April. The Gator Farm and other attractions will be available. You just won't get the full MB experience.
When my kids were younger like the OP's, we went to MB a few times in April simply for the warmer weather. We made the most of the activities available and had a great time.
For true beach weather I'd need to go further south which is too far to drive on a Spring Break schedule.
Now that my kids are older we travel to the Shenandoah Valley which is not as warm,but has lots of activities and is much closer to me than MB.
As my camping experience has developed I have learned to maximise my drive time vs. relaxation time ratio.
While the drive is generally enjoyable and part of the experience,the real goal is not driving but exploring,camping and relaxing.
If there is a choice, I now give more consideration to the closer destination.
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