It is advisable to keep pets in whilst applying the sealer and for an hour or two thereafter as contact with uncured sealer can cause irritation. Once the sealer has dried it will have no harmful effects on people or pets. Avoid spilling or spraying into fishponds as solvents are detrimental to pond life (especially fish).
If you are using a water based seal then it is not really a problem as the sealer is non hazardous and will only cause a mild aggravation to the animal. If however you are using a solvent based sealer then this can cause more of an issue.. The best way to avoid this is to keep and pets indoors until the sealer has fully cured!