I agree fostering can be a huge positive for birth children, but just want to play devils advocate and highlight some of the downsides.
Most children coming into care at the moment are older children, or sibling groups - I foster for my local authority (a large one) and I think only 2 babies have been removed this year unless part of a sibling group. Would your child feel put out no longer being number one?
Likewise, respite is rare at the moment - no idea why as FCs here aren't paid when a child is in respite, and there are lots of carers without placements who would be happy to assist. My LA haven't recruited respite carers for a long time now.
You need to consider that lots of family things you currently do may have to change - e.g, you can't bath a foster child with your own children; dressing gowns have to be worn (by all children and adults) over PJs; you can't have a foster child into your bed to watch TV or Sunday morning snuggles - if you do this with your own kids you need to consider how a FC will feel not being included.
Also, even the youngest children come with problems. I never, ever leave a FC unattended with other children (even to pop out of the room for a second) as if a foster child makes an allegation against your child it WILL be taken seriously. Plus even after many months foster children can display unknown behaviour (especially as they become more secure).
A large proportion of the week is taken up by contact - usually around 3 times a week but can be more. You don't often have a say as to when this takes place. If it's smack bang in the middle of your child's nap time you'll have to work round that. If it's your child's nursery time you'll have to work round that. Contact centres are usually calm, well managed places but they can also be chaotic, full of angry parents with emotions running high. The centre I take my FC to have the police on speed dial - and yes the number is used!
I don't want to put you off - I love fostering - but it's far from easy, especially with young birth children.