For the bike, just make sure it is comfortable to ride and easy to get on and off. With a little one in tow this is really important. You will be happier with larger cogs on the rear as carting kids around needs low ratio gears, not like the road bikes. The child's helmet should have a flat back, not a pointy back like adult helmets. The flat back means that their head will not be pushed forward by the helmet.
Child seats on the rear can make the bike top heavy and unstable. If the toddler does not want to get in, it can be almost impossible to strap them in and keep the bike upright, even with a stand. Once the seat is one, you have no space for panniers and baby cops your backpack in their face. Plus there is no space for their bag and stuff.
I am biased against them. I caught the wheel of a mother & child in one of the big bike events. I went down and she stayed upright through luck. She then mentioned how exposed the child's limbs are. Her husband had fallen and baby had got his arm caught between the road and the seat. It broke in two places and needed two operations.
After our bingle she swore off them forever and decided to buy a trailer instead.
A trailer is more pricey - although Kmart has cheap ones at the moment, just over $120. They are more stable and there is a roll cage of aluminium frame around the child. My daughter hates wearing her helmet in it so that can be a battle. They can get hot inside too, so you need to make sure there is ventilation - like a window flap. Trailers have the advantage that you can take them off and your bike is back to normal. I can take my child to daycare, leave the trailer and go on to work with my bike and panniers at the ready. The trailer has a boot plus space inside for a toy, drink, blanket, daycare pack and so on. You can get some really speccy ones - with reclining seats, shaped seats for pointy-backed helmets and UV treated windows. They are over $1000, mine was $250 about two years ago. It is a Pacific one from the wholesaler in Campbellfield.
Finally, I have to agree with Tim. I'd love a cargo bike but cannot justify one until my trailer breaks... My trailer is mostly used for grocery shopping so hopefully this will not be too long!