You can carry children on your bike or trike in a custom-made child seat or tow them in a trailer. Read this comprehensive advice guide on child seats (note that there is a printer-friendly version). You can also tow children in a trailer or on a trailer bike, as discussed here. There is also a brilliant article on icebike.org.
Other options are to use a Bakfiets or Larry vs Harry, buy or adapt a tandem or tricycle. If you have any questions on cycling with children, it's a good idea to ask for advice on a cycling forum or site such as Netmums.
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