British Passport or Belgian I.D card?

I need to renew my kids passport but am thinking about whether it's an easier option if i just get them a Belgian i.d card. They currently have British passports (British Citizens).
We need to travel to the UK in july.
Will having a belgian i.d affect their british nationality or are they dual nationality?
Once the British passport expires how can I prove they are British. They were born in Spain. British mother, Belgian father.
Thanks in advance