slugga said..
Mate first board buy a cheap basic one because after a few months you will want a upgrade the progression of paddling is amazing and as your paddling skills improve so does your knowledge of boards and paddles and you will work out what you like and need in a board.. Most guys in this forum would have several boards for different situations and conditions.. Put it this way . I started supping nearly exactly a year ago. I have had 4 different boards in that year and currently still have 3 that I use .. I started on a 10"2 170liter big board that we use as the family board or when the surf is tiny then went to a 8"10 wide point that I sold to get a 8"3 x32 naish and I got a 9"2 Jp surf for bigger waves . So to start off get one that's a basic family board try looking on the buy and sell section but you will be looking for a new board for your self in no time as this sport is really addictive .. Good luck
+1
At Xmas it will be 2 years for me and I have 6 boards. OK, I may have an illness

but as slugga says, you will change quickly.
I had the same stats as you when I started and I bought a 10' board. Bought a 9'5" x 32" after a month and the next board came after 7 months. My 8'8" X32 board was a bit of a challenge at first so I lost 10 kg and now it is easier to ride. Now I have a 7'10" and need to lose more weight

But for river paddling with your child, a 10' board will be good.