I'm thinking that space is going to be really tight in an economy car with your family and luggage situation. I think that you will need to upgrade.
The rental agency will usually have a chart that tells you what the capacity of the vehicles in the reserved class will hold. These charts are pretty accurate.
Most economy cars fit 4 adults OK, the company may say it holds 5, but you need to be 3 children or 3 very friendly adults to fit in the rear seat. The luggage is normally 1 large, 1 medium and 1 small suitcase You will also need a carseat for your 1 year old, you can rent one from the rental company.