We've been on a junk three times with our children. The first time, nearly five years ago, we just had one 18 mth old - he tottered round, a bit unsteady at time if it got choppy and we chased round after him. We rocked him to sleep on the boat and the engine noise and the motion of the boat helped with that. There was just us and one other family so it was easy to keep an eye on him at all times.
The second time, earlier this summer, there were about 10 families. We had a 5-yr-old, 3-yr-old and 9-mth-old - and frankly, it was a nightmare. I couldn't relax at all, trying to keep track of the two bigger ones and entertain the 9-mth-old. And because there were a lot of other kids on board, it was a bit chaotic. The baby did fall asleep in his pushchair in the air-conditioned cabin, so at least he took his nap.
The third time, also this summer, there were only 3 families so it was easier to watch the children. We also took our helper, so she could follow the two bigger boys around the whole time, while my husband and I played pass-the-baby. It was much easier and we had a very enjoyable time.
So, are junks child-friendly? Well, you have to be very vigilant - watch them on the stairs and near the rails, get them to wear a floatie vest for the whole trip if they will - but they motion in the water isn't usually too rough (bit choppy in the harbour). Make sure you have a junk with a cabin where the kids can hide from the sun. Better yet, a closed, air-conditioned cabin - and take a sling to carry the baby for a while, and a pushchair for a nap. We also took a portable DVD player as it's a long trip up to Sai Kung and the kids get bored with the scenery and exploring the boat. For the journey back, the DVD player was a godsend.
I would think a 7-mth-old would be reasonably easy - not walking, not very mobile, happy to be carried (and light enough to be in the sling for extended periods). I would say go for it - you'll probably have fun. We don't drink much if we're watching the kids (a couple of glasses of wine at the most) but it was still a fun trip. It's a Hong Kong memory that's worth having!
HTH