We have made about 25 trips to Venice over the years some of which were stays of several weeks so we know it fairly well. We do not recommend staying near P. Roma. As has been suggested, take a water taxi to a hotel near San Marco as you will have a much better / more interesting experience. I agree with the poster who recommended a water taxi. They are expensive however, my guess is your grandkids will remember it as a highlight and when you consider that it will take all of you and your luggage the price isn’t too bad. If the weather is good the back of the taxi will be open and you can stand up and get an amazing view of the Grand Canal if that’s the route you go.
If you do this it helps to book a hotel with a water entrance as then you will dock right there and the hotel staff can help with your bags going to your room. But if you arrive during an aqua alta (high tide that floods the city) it might not be possible to get right to your hotel as at times the water is too high for boats to get under the bridges ( not often, but it can happen ).
Hope you have a great time! It’s a magical place especially in San Marco after sunset listening to the palm court orchestras at the cafes.