While there are going to be toys already at the place, new toys can be exciting and will keep the children interested for the visit thanks to the novelty. You may need to ask the parents first, to be sure that the toy isn't one the children already own.
Babies would enjoy rattles and anything that shakes. They would also like mobiles or activity gyms, activity quilts, plush toys, or bright baby toys. Toys that squeak such as play cubes or soft balls are other toys for babies. Toddlers (1-3 year olds) would like baby dolls, bubbles, any toy that is colorful and looks real, or pretend vehicles designed for toddlers. Girls 4-8 would probably like dress up and makeover things. Some crafts that are not to detailed but more than coloring work as well. Bring over some simple card and board games designed for this age. 9+ is a bit more trickery but most of the time kids that age like Bingo and other classic board games. They also like friends and girls will probably like more sophisticated crafts that are detailed. Children this age also love age-appropriate video games, bring a few over, though you should be sure they do not spend more than an hour of your babysitting time playing them. Tomboys would like footballs and soccer balls. Toddler boys would probably enjoy Duplo blocks. 4-8 year olds might like car toys, trucks, etc. Apps on your iPad might prove fun for the kids if they haven't seen your games or educational apps yet.