Well, I think the answer is "Yes" and "No".
On the "Yes"-side, there does appear to be evidence that students lose some level of knowledge during the summer article. But, on the "No" side, it looks like this loss may be exaggerated another article. Also, the "loss" appears to occur more often among poor kids than among middle-class kids (see graphic below).
Also on the "No" side, there are 50 million students in P-12 education in the United States and a good majority of them seem to do OK year to year. So, if there is a summer loss, Is this something that we should worry about? I think that we all want the best for our children and we all want our children to do their best at all times. We want them to remember what they have learned; we want them to grow physically and academically. We don't want them to struggle. If your child is required to do a lot of reading during the school year and then he/she does no reading during the summer, it makes sense that some loss of ability is going to occur.
No parent would ever allow their child to go without vegetables for ten weeks during the summer. So don't allow your child to go without exercising and reading and doing math and thinking and creating and discovering and learning during the summer either. It's not that we are afraid that they will get a bad grade in September. It is that we want our children to grow all of the time. That includes academic growth.
During the summer, the parents are the teachers and the home is the school. Keep learning!
For ideas to keep up your skills in mathematics during the summer, check out our Summer Math site.