#1 The more powerful 4-wheelers usually have two shifts (low and high) so it won't be too fast for little ones as long as your yard is flat or you go to a park. For the twins, why don't you just get one that looks like a truck or car so they can ride together and take turns behind the wheel? So you can invest in a better model with more bells and whistles.
#2 It is a fun choice and they will love it then they get older. So, patience :-)
#3 24 inch bike is too high for 5-y-o. 16 inch is good for 5-y-o. So if your twins are wearing 4T already, 16 inch bike should work and you can always lower the seat. With training wheels they really won't fall that easily.
BTW, helmet, gloves, and kneels/elbow protections are must at all times.
Good luck and happy holidays.
#2 It is a fun choice and they will love it then they get older. So, patience :-)
#3 24 inch bike is too high for 5-y-o. 16 inch is good for 5-y-o. So if your twins are wearing 4T already, 16 inch bike should work and you can always lower the seat. With training wheels they really won't fall that easily.
BTW, helmet, gloves, and kneels/elbow protections are must at all times.
Good luck and happy holidays.