Yes, but they can do so very easily. If needed. Fast tracked legislation can get through the Commons and Lords in a single day if needed (and has done so in the past, in relation to, for example, nationalising Northern Rock).
Also in relation to whether Kate would be a suitable Regent - it’s worth bearing in mind that this hypothetical situation only comes into play if Charles is dead and William is King, but dies before George is 18. In which case, Kate would already be the Queen, and with probably more awareness of the reality of the role than other members of William’s families.