Class M7 from Alexandra School sent in this solution which they worked on together:

First we added one brother and checked whether the statements for Sally and Mark were true. Then we added one brother ... another sister ... another brother ... until we found an arrangement that worked. These are our results:

Table of results for Brown family
Boys	Girls	True for Sally	True for Mark
1	2	Yes	No
2	2	No	No
2	3	Yes	No
3	3	No	No
3	4	Yes	Yes

So there are three boys and four girls in the Brown family.

They added:

We really enjoyed being able to click to add another brother or sister appear. It helped us to work systematically through the puzzle and find the solution.

This is very clearly presented, well done. We had many other solutions which used the interactivity too, and some from people who drew pictures themselves, which is just as good.